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Versions: (draft-thubert-6lo-rfc6775-update) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21

6lo                                                      P. Thubert, Ed.
Internet-Draft                                                     Cisco
Updates: 6775 (if approved)                                  E. Nordmark
Intended status: Standards Track                                  Zededa
Expires: December 21, 2018                                S. Chakrabarti
                                                                 Verizon
                                                              C. Perkins
                                                               Futurewei
                                                           June 19, 2018


         Registration Extensions for 6LoWPAN Neighbor Discovery
                    draft-ietf-6lo-rfc6775-update-21

Abstract

   This specification updates RFC 6775 - 6LoWPAN Neighbor Discovery, to
   clarify the role of the protocol as a registration technique,
   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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on December 21, 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



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   (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.  BCP 14  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  References  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Acronym Definitions . . . . . . . . . . . . . . . . . . .   4
     2.4.  New Terms . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Applicability of Address Registration Options . . . . . . . .   6
   4.  Extended Neighbor Discovery Options and Messages  . . . . . .   7
     4.1.  Extended Address Registration Option (EARO) . . . . . . .   7
     4.2.  Extended Duplicate Address Message Formats  . . . . . . .  11
     4.3.  Extensions to the Capability Indication Option  . . . . .  12
   5.  Updating RFC 6775 . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Extending the Address Registration Option . . . . . . . .  14
     5.2.  Transaction ID  . . . . . . . . . . . . . . . . . . . . .  16
       5.2.1.  Comparing TID values  . . . . . . . . . . . . . . . .  16
     5.3.  Registration Ownership Verifier (ROVR)  . . . . . . . . .  17
     5.4.  Extended Duplicate Address Messages . . . . . . . . . . .  19
     5.5.  Registering the Target Address  . . . . . . . . . . . . .  19
     5.6.  Link-Local Addresses and Registration . . . . . . . . . .  20
     5.7.  Maintaining the Registration States . . . . . . . . . . .  21
   6.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .  23
     6.1.  Signaling EARO Support  . . . . . . . . . . . . . . . . .  23
     6.2.  RFC6775-only 6LN  . . . . . . . . . . . . . . . . . . . .  24
     6.3.  RFC6775-only 6LR  . . . . . . . . . . . . . . . . . . . .  24
     6.4.  RFC6775-only 6LBR . . . . . . . . . . . . . . . . . . . .  24
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  26
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     9.1.  ARO Flags . . . . . . . . . . . . . . . . . . . . . . . .  27
     9.2.  EARO I-Field  . . . . . . . . . . . . . . . . . . . . . .  28
     9.3.  ICMP Codes  . . . . . . . . . . . . . . . . . . . . . . .  28
     9.4.  New ARO Status values . . . . . . . . . . . . . . . . . .  29
     9.5.  New 6LoWPAN Capability Bits . . . . . . . . . . . . . . .  30
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  31
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     11.2.  Terminology Related References . . . . . . . . . . . . .  32
     11.3.  Informative References . . . . . . . . . . . . . . . . .  32



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     11.4.  External Informative References  . . . . . . . . . . . .  35
   Appendix A.  Applicability and Requirements Served (Not
                Normative) . . . . . . . . . . . . . . . . . . . . .  36
   Appendix B.  Requirements (Not Normative) . . . . . . . . . . . .  37
     B.1.  Requirements Related to Mobility  . . . . . . . . . . . .  37
     B.2.  Requirements Related to Routing Protocols . . . . . . . .  38
     B.3.  Requirements Related to the Variety of Low-Power Link
           types . . . . . . . . . . . . . . . . . . . . . . . . . .  39
     B.4.  Requirements Related to Proxy Operations  . . . . . . . .  40
     B.5.  Requirements Related to Security  . . . . . . . . . . . .  40
     B.6.  Requirements Related to Scalability . . . . . . . . . . .  42
     B.7.  Requirements Related to Operations and Management . . . .  42
     B.8.  Matching Requirements with Specifications . . . . . . . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

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

   This specification updates 6LoWPAN ND to simplify and generalizes
   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  Determine the most recent location in case of node mobility

   o  Simplify the registration flow for Link-Local Addresses

   o  Support a routing-unaware Leaf Node in a Route-Over network

   o  Proxy registration in a Route-Over network

   o  Enable verification for the registration, using the Registration
      Ownership Verifier (ROVR)

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

   o  Better support for privacy and temporary addresses

   These features satisfy requirements as listed in Appendix B.




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2.  Terminology

2.1.  BCP 14

   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.  References

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

   o  "Neighbor Discovery for IP version 6" [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], and

   o  "Neighbor Discovery Optimization for Low-power and Lossy Networks"
      [RFC6775],

2.3.  Acronym Definitions

   This document uses the following acronyms:

   6BBR: 6LoWPAN Backbone Router

   6LBR: 6LoWPAN Border Router

   6LN:  6LoWPAN Node

   6LR:  6LoWPAN Router

   6CIO: Capability Indication Option

   EARO: (Extended) Address Registration Option -- (E)ARO

   EDAR: (Extended) Duplicate Address Request -- (E)DAR

   EDAC: (Extended) Duplicate Address Confirmation -- (E)DAC




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   DAD:  Duplicate Address Detection

   DODAG:  Destination-Oriented Directed Acyclic Graph

   LLN:  Low-Power and Lossy Network

   NA:   Neighbor Advertisement

   NCE:  Neighbor Cache Entry

   ND:   Neighbor Discovery

   NDP:  Neighbor Discovery Protocol

   NS:   Neighbor Solicitation

   ROVR: Registration Ownership Verifier (pronounced rover)

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

   RA:   Router Advertisement

   RS:   Router Solicitation

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

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 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 Extended ARO option.

   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.




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   Registered Address:  An address registered for the Registered Node.

   RFC6775-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 provided at the
         IP layer.

   Routing Registrar:  An IPv6 ND Registrar that also provides
         reachability services for the Registered Address, including
         Duplicate Address Detection and proxy Neighbor Advertisement.

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

   updated:  A 6LN, a 6LR, or a 6LBR that supports this specification,
         in contrast to an RFC6775-only device.

3.  Applicability of Address Registration Options

   The Address Registration Option (ARO) in [RFC6775] facilitates
   Duplicate Address Detection (DAD) for hosts and populates Neighbor
   Cache Entries (NCEs) [RFC4861] in the routers.  This reduces the
   reliance on multicast operations, which are often as intrusive as
   broadcast, in IPv6 ND operations (see
   [I-D.ietf-mboned-ieee802-mcast-problems]).

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

   o  the router running out of space;

   o  a registration bearing a stale sequence number which 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 [RFC4291];

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

   In such cases the host will receive an error to help diagnose the
   issue and may retry, possibly with a different address, and possibly
   registering to a different router, depending on the returned error.



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   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 "Privacy Extensions for
   Stateless Address Autoconfiguration (SLAAC) in IPv6" [RFC4941], and
   SHOULD conform to "Host Address Availability Recommendations"
   [RFC7934].

   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 6LoWPAN Router (6LR) or
   6LoWPAN Border Router (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 6LR/6LBRs in their network support
   the number and type of devices that can register to them, based on
   the number of IPv6 addresses that those devices require and their
   address renewal rate and behavior.

4.  Extended Neighbor Discovery Options and Messages

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

4.1.  Extended Address Registration Option (EARO)

   The Address Registration Option (ARO) is defined in section 4.1 of
   [RFC6775].

   This specification introduces the Extended Address Registration
   Option (EARO) based on the ARO for use in NS and NA messages.  The
   EARO includes a sequence counter called Transaction ID (TID) that is
   used to determine the latest location of a registering mobile device.
   A new 'T' flag indicates 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 in order to carry
   different types of information, e.g., cryptographic information of
   variable size.  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
   of the option minus 1.




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   Section 5.1 discusses those changes in depth.

   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 Option 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
                   zero when not used.

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

   I:              Two-bit Integer: A value of zero 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.






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   R:              The Registering Node sets the 'R' flag to request
                   reachability services for the registered address from
                   a Routing Registrar.

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

   TID:            One-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 a same IPv6
                   Address.  The ROVR size MUST be 64 bits when backward
                   compatibility is needed; otherwise the size MAY be
                   128, 192, or 256 bits.





























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   +-------+-----------------------------------------------------------+
   | Value | Description                                               |
   +-------+-----------------------------------------------------------+
   |  0..2 | As defined in [RFC6775].  Note: a Status 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 a 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 source of   |
   |       | the NS(EARO) conflicts with an existing registration.     |
   |       |                                                           |
   |   7   | Invalid Source Address: The address used as 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




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4.2.  Extended Duplicate Address Message Formats

   The DAR and DAC messages share a common base format as defined in
   section 4.4 of [RFC6775].  Those messages enable information from the
   ARO to be transported over multiple hops.  The DAR and DAC are
   extended as shown in Figure 2:


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |CodePfx|CodeSfx|          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Status     |     TID       |     Registration Lifetime     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
     ...            Registration Ownership Verifier (ROVR)           ...
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                       Registered Address                      +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 2: Duplicate Address Messages Format

   Modified Message Fields:

   Code:           The ICMP Code [RFC4443] for Duplicate Address
                   Messages is split in two 4-bit fields, the Code
                   Prefix and the Code Suffix.  The Code Prefix MUST be
                   set to zero 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, and 256
                   bits, respectively.

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





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

4.3.  Extensions to the Capability Indication Option

   This specification defines 5 new capability bits for use in the 6CIO,
   defined by [RFC7400] for use in IPv6 ND messages.

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

   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 6LR, 6LBR, and Routing Registrar (e.g., 6BBR),
   respectively.  These flags are not mutually exclusive; an updated
   node can advertise multiple collocated functions.


       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

   Option Fields:

   Type:  36

   L: Node is a 6LR.

   B: Node is a 6LBR.

   P: Node is a Routing Registrar.

   E: Node is an IPv6 ND Registrar -- i.e., it supports registrations
      based on EARO.



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   D: 6LBR supports EDAR and EDAC messages.

5.  Updating RFC 6775

   The Extended Address Registration Option (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 Neighbor
   Discovery, 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, 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 option are the Duplicate Address
   Request (DAR) and Duplicate Address Confirmation (DAC), used in the
   Duplicate Address messages.  They convey the additional information
   all the way to the 6LBR.

   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).












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                                               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 for
   privacy / 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 RFC6775-only one; otherwise
   the Registering Node operates in a backward-compatible fashion when
   attaching to an RFC6775-only 6LR.

5.1.  Extending the Address Registration Option

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

   The Neighbor Solicitation (NS) and the ARO are modified as follows:





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   o  The Target Address 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 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 also
      avoids in most cases the use of an address as source address
      before it is registered.

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

   o  The option Length 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 of size corresponds to a larger ROVR field, so
      the size of the ROVR is inferred from the option Length.

   o  A new Opaque field is introduced to carry opaque information in
      case 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 Zero for I
      indicates topological information to be passed to a routing
      process if the registration is redistributed.  In that case, a
      value of Zero for the Opaque field is backward-compatible with the
      reserved fields that are overloaded, and the meaning is to use the
      default topology.

   o  This document specifies a new flag in the EARO, the 'R' flag.  If
      the 'R' flag is set, the Registering Node requests the 6LR to
      ensure reachability for the Registered Address, e.g., by means of
      routing or proxying 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]).

   o  A node that supports this specification MUST be provide a
      Transaction ID (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).

   A 6LN that acts only as a host, when registering, 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.



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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
   Appendix A and Section 5.7).

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 the "Sequence Counter
   Operation" section of the "IPv6 Routing Protocol for Low-Power and
   Lossy Networks" [RFC6550] specification.

   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 used as a circular sequence number space of size
   128 as 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.

   For a given sequence counter,

   1.  The sequence counter SHOULD be initialized to an implementation
       defined value which is 128 or greater prior to use.  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 zero.  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.




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   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 [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.

   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



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   Node, such as the EUI-64 address of an interface.  This can also be a
   token obtained with cryptographic methods which 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 (more
   in Section 5.6).  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 used to prove the ownership of the
   registration as specified in "Address Protected Neighbor Discovery
   for Low-power and Lossy Networks" [I-D.ietf-6lo-ap-nd].  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 on ROVR collision: different techniques for forming the ROVR
   will operate in different name-spaces.  [RFC6775] operates on EUI-
   64(TM) addresses.  [I-D.ietf-6lo-ap-nd] generates cryptographic
   tokens.  While collisions are not expected in the EUI-64 name-space
   only, they may happen in the case of [I-D.ietf-6lo-ap-nd] and in a
   mixed situation.  An implementation that understands the name-space
   MUST consider that ROVRs from different name-spaces are different
   even if they have the same value.  An RFC6775-only 6LR or 6LBR will
   confuse the name-spaces, which slightly increases the risk of a ROVR
   collision.  A collision of ROVR 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
   registration for that IPv6 address.  Also, when the ROVR is not an
   EUI-64 address, then it MUST NOT be used as the interface ID 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 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 the 6LRs and the 6LBR time out
   the previous registration, or a management action is taken to clear
   the relevant state in the network.



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5.4.  Extended Duplicate Address Messages

   In order to map the new EARO content in the Extended Duplicate
   Address (EDA) messages, a new TID field is added to the Extended DAR
   (EDAR) and the Extended DAC (EDAC) messages as a replacement of 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 Extended Duplicate Address messages are
   backward compatible with the RFC6775-only versions as long as the
   ROVR field is 64 bits long.  Remarks concerning backwards
   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 [RFC6775].  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 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 Source Link Layer Address (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 mesh, 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 mesh
   from the Registering Node, the SLLA in the NS(ARO) is 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 indicates the link-layer address of the 6LN that owns the
   address.





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   A Registering Node (e.g., a 6LBR also acting as 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 RFC6775-only 6LoWPAN ND [RFC6775].

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 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 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.

   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 a 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
   and the Target Address of an NS(EARO) message.  In that case, it is



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   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 the "Optimistic Duplicate Address Detection
   (ODAD) for IPv6" [RFC4429].  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 Source Link-Layer 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 Source Address, in contrast to a node that complies
   to [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 of this document.

   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" (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 for the node to retry this registration via another 6LR, since



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   the problem is network-wide.  The node may either abandon that
   address, de-register other addresses first to make room, or keep the
   address in TENTATIVE state 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 till the current Registration
   Lifetime expires and this may lead to a situation where the 6LR
   resources become saturated, even if they are 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
   [I-D.ietf-roll-efficient-npdao]).  Whether it could do so or not, the
   receiver MUST clean up said state.

   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 evaluates 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 of "Success" and the entry is scheduled to be removed.
   Otherwise, a Status Code of "Moved" is returned instead, and the
   existing entry is maintained.




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   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 protect a mobile node that de-registered from one 6LR and did
   not register yet to a new one, or the new registration did not yet
   reach the 6LBR due to propagation delays in the network.  Once the
   DELAY time is 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].
   On the contrary, 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 is
   backward-compatible since the 'T' flag and TID field are reserved in
   [RFC6775], and are ignored by an RFC6775-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,
   which has no consequence if the Registered Addresses are different.

6.1.  Signaling EARO Support

   "Generic Header Compression for IPv6 over 6LoWPANs" [RFC7400]
   specifies the 6LoWPAN Capability Indication Option (6CIO) to indicate
   a node's capabilities to its peers.  The 6CIO MUST be present in both
   Router Solicitation (RS) and Router Advertisement (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 support for
   EARO 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



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   response to an RS with a 6CIO indicating the capability to act as a
   6LR, since the RA propagates information between routers.

6.2.  RFC6775-only 6LN

   An RFC6775-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 RFC6775-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.  RFC6775-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 a RFC6775-only 6LR.

   An updated 6LN MUST use an EARO in the request regardless of the type
   of 6LR, RFC6775-only or updated, which implies that the 'T' flag is
   set.  It MUST use a ROVR of 64 bits if the 6LR is an RFC6775-only
   6LR.

   If an updated 6LN moves from an updated 6LR to an RFC6775-only 6LR,
   the RFC6775-only 6LR will send an RFC6775-only DAR message, which
   cannot be compared with an updated one for recency.  Allowing
   RFC6775-only DAR messages to update a state established by the
   updated protocol in the 6LBR would be an attack vector and that
   cannot be the default behavior.  But if RFC6775-only and updated 6LRs
   coexist temporarily in a network, then it makes sense for an
   administrator to install a policy that allows this, using some method
   out of scope for this document.

6.4.  RFC6775-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 RFC6775-only 6LBR will accept and process an EDAR
   message as if it were an RFC6775-only DAR, as long as the ROVR is 64
   bits long.  An updated 6LR discovers the capabilities of the 6LBR in




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   the 6CIO in RA messages from the 6LR; if the 6CIO was not present in
   any RA, then the 6LBR is assumed to be a RFC6775-only 6LBR.

   If the 6LBR is RFC6775-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 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 a
   lower layer encryption to defeat potential attacks.  This
   specification requires the LLN MAC 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 Neighbor Discovery messages.

   This specification recommends using privacy techniques (see
   Section 8), and protecting against address theft by methods outside
   the scope of this document.  As an example, "Address Protected
   Neighbor Discovery for Low-power and Lossy Networks"
   [I-D.ietf-6lo-ap-nd] guarantees the ownership of the Registered
   Address using a cryptographic ROVR.

   The registration mechanism may be used by a rogue node to attack the
   6LR or the 6LBR with a Denial-of-Service attack against the registry.
   It may also happen that the registry of a 6LR or a 6LBR is saturated
   and cannot take any more registrations, which effectively denies the
   requesting node the capability to use a new address.  In order to
   alleviate those concerns, 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.  See
      Section 5.2 for the mechanism to avoid replay attacks and avoiding
      the use of stale registration information.

   o  The Registration lifetimes SHOULD be individually configurable for
      each address or group of addresses.  The nodes SHOULD be
      configured with a Registration Lifetime that reflects their
      expectation of how long they will use the address with the 6LR to
      which it is registered.  In particular, use cases that involve
      mobility or rapid address changes SHOULD use lifetimes that are



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      larger yet of a same order as the duration of the expectation of
      presence.

   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 for the 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
      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.  This trust model could be at a minimum based
   on a Layer-2 access control, or could provide role validation as well
   (see Req5.1 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.



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

   "Privacy Considerations for IPv6 Adaptation-Layer Mechanisms"
   [RFC8065] 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,
   then privacy concerns should be considered.  In particular, new
   implementations should follow the IETF "Recommendation on Stable IPv6
   Interface Identifiers" [RFC8064].  [RFC8064] recommends the use of "A
   Method for Generating Semantically Opaque Interface Identifiers with
   IPv6 Stateless Address Autoconfiguration (SLAAC)" [RFC7217] for
   generating Interface Identifiers to be used in SLAAC.

9.  IANA Considerations

      Note to RFC Editor, to be removed: please replace "This RFC"
      throughout this document by the RFC number for this specification
      once it is allocated.

   IANA is requested to make a number of changes under the "Internet
   Control Message Protocol version 6 (ICMPv6) Parameters" registry, as
   follows.

9.1.  ARO Flags

   IANA is requested to create a new subregistry for "ARO Flags" under
   the "Internet Control Message Protocol version 6 (ICMPv6) [RFC4443]
   Parameters".

   This specification defines 8 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 policy is "IETF Review" or "IESG Approval" [RFC8126].

   The initial content of the registry is as shown in Table 2.






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                +-------------+--------------+-----------+
                |  ARO Status | Description  | Document  |
                +-------------+--------------+-----------+
                |     0..5    | Unassigned   |           |
                |             |              |           |
                |      6      | 'R' Flag     | This RFC  |
                |             |              |           |
                |      7      | 'T' Flag     | This RFC  |
                +-------------+--------------+-----------+

                          Table 2: New ARO Flags

9.2.  EARO I-Field

   IANA is requested to create a new subregistry for "ARO Flags" under
   the "Internet Control Message Protocol version 6 (ICMPv6) [RFC4443]
   Parameters".

   This specification defines 4 integer values from 0 to 3, and assigns
   value 0 (see Section 4.1).  The policy is "IETF Review" or "IESG
   Approval" [RFC8126].

   The initial content of the registry is as shown in Table 3.

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

              Table 3: New subregistry for the EARO "I" Field

9.3.  ICMP Codes

   IANA is requested to create 2 new subregistries of the ICMPv6 "Code"
   Fields registry, which itself is a subregistry of the Internet
   Control Message Protocol version 6 (ICMPv6) Parameters for the ICMP
   codes.

   The new subregistries relate to the ICMP type 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 2 subfields, the "Code Prefix" and the "Code
   Suffix".  The new subregistries relate to the "Code Suffix" portion
   of the ICMP Code.  The range of "Code Suffix" is 0..15 in all cases.




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   The policy is "IETF Review" or "IESG Approval" [RFC8126] for both
   subregistries.

   The new subregistries are to be initialized as follows:

   +--------------+--------------------------------------+------------+
   | Code Suffix  | Meaning                              | Reference  |
   +--------------+--------------------------------------+------------+
   | 0            | RFC6775 DAR message                  | RFC 6775   |
   |              |                                      |            |
   | 1            | EDAR message with 64-bit ROVR field  | This RFC   |
   |              |                                      |            |
   | 2            | EDAR message with 128-bit ROVR field | This RFC   |
   |              |                                      |            |
   | 3            | EDAR message with 192-bit ROVR field | This RFC   |
   |              |                                      |            |
   | 4            | EDAR message with 256-bit ROVR field | This RFC   |
   |              |                                      |            |
   | 5...15       | Unassigned                           |            |
   +--------------+--------------------------------------+------------+

         Table 4: New Code Suffixes for ICMP type 157 DAR message

   +--------------+--------------------------------------+------------+
   | Code Suffix  | Meaning                              | Reference  |
   +--------------+--------------------------------------+------------+
   | 0            | RFC6775 DAC message                  | RFC 6775   |
   |              |                                      |            |
   | 1            | EDAC message with 64-bit ROVR field  | This RFC   |
   |              |                                      |            |
   | 2            | EDAC message with 128-bit ROVR field | This RFC   |
   |              |                                      |            |
   | 3            | EDAC message with 192-bit ROVR field | This RFC   |
   |              |                                      |            |
   | 4            | EDAC message with 256-bit ROVR field | This RFC   |
   |              |                                      |            |
   | 5...15       | Unassigned                           |            |
   +--------------+--------------------------------------+------------+

         Table 5: New Code Suffixes for ICMP type 158 DAC message

9.4.  New ARO Status values

   IANA is requested to make additions to the Address Registration
   Option Status Values Registry as follows:






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   +-------------+-----------------------------------------+-----------+
   |  ARO Status | Description                             | Document  |
   +-------------+-----------------------------------------+-----------+
   |      3      | Moved                                   | This RFC  |
   |             |                                         |           |
   |      4      | Removed                                 | This RFC  |
   |             |                                         |           |
   |      5      | Validation Requested                    | This RFC  |
   |             |                                         |           |
   |      6      | Duplicate Source Address                | This RFC  |
   |             |                                         |           |
   |      7      | Invalid Source Address                  | This RFC  |
   |             |                                         |           |
   |      8      | Registered Address topologically        | This RFC  |
   |             | incorrect                               |           |
   |             |                                         |           |
   |      9      | 6LBR Registry saturated                 | This RFC  |
   |             |                                         |           |
   |      10     | Validation Failed                       | This RFC  |
   +-------------+-----------------------------------------+-----------+

                      Table 6: New ARO Status values

9.5.  New 6LoWPAN Capability Bits

   IANA is requested to make additions to the Subregistry for "6LoWPAN
   Capability Bits" as follows:

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

                   Table 7: New 6LoWPAN Capability Bits








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10.  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 Kuhlewind, Ben Campbell, Eric
   Rescorla, and Lorenzo Colitti for their various contributions and
   reviews.  Also, many thanks to Thomas Watteyne for the world first
   implementation of a 6LN that was instrumental to the early tests of
   the 6LR, 6LBR and Backbone Router.

11.  References

11.1.  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>.

   [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>.

   [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>.






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   [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>.

11.2.  Terminology Related References

   [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>.

   [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>.

11.3.  Informative References

   [I-D.chakrabarti-nordmark-6man-efficient-nd]
              Chakrabarti, S., Nordmark, E., Thubert, P., and M.
              Wasserman, "IPv6 Neighbor Discovery Optimizations for
              Wired and Wireless Networks", draft-chakrabarti-nordmark-
              6man-efficient-nd-07 (work in progress), February 2015.

   [I-D.delcarpio-6lo-wlanah]
              Vega, L., Robles, I., and R. Morabito, "IPv6 over
              802.11ah", draft-delcarpio-6lo-wlanah-01 (work in
              progress), October 2015.





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   [I-D.hou-6lo-plc]
              Hou, J., Hong, Y., and X. Tang, "Transmission of IPv6
              Packets over PLC Networks", draft-hou-6lo-plc-03 (work in
              progress), December 2017.

   [I-D.ietf-6lo-ap-nd]
              Thubert, P., Sarikaya, B., and M. Sethi, "Address
              Protected Neighbor Discovery for Low-power and Lossy
              Networks", draft-ietf-6lo-ap-nd-06 (work in progress),
              February 2018.

   [I-D.ietf-6lo-backbone-router]
              Thubert, P., "IPv6 Backbone Router", draft-ietf-6lo-
              backbone-router-06 (work in progress), February 2018.

   [I-D.ietf-6lo-nfc]
              Choi, Y., Hong, Y., Youn, J., Kim, D., and J. Choi,
              "Transmission of IPv6 Packets over Near Field
              Communication", draft-ietf-6lo-nfc-09 (work in progress),
              January 2018.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-14 (work
              in progress), April 2018.

   [I-D.ietf-mboned-ieee802-mcast-problems]
              Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
              Zuniga, "Multicast Considerations over IEEE 802 Wireless
              Media", draft-ietf-mboned-ieee802-mcast-problems-01 (work
              in progress), February 2018.

   [I-D.ietf-roll-efficient-npdao]
              Jadhav, R., Thubert, P., Sahoo, R., and Z. Cao, "Efficient
              Route Invalidation", draft-ietf-roll-efficient-npdao-03
              (work in progress), March 2018.

   [I-D.struik-lwip-curve-representations]
              Struik, R., "Alternative Elliptic Curve Representations",
              draft-struik-lwip-curve-representations-00 (work in
              progress), October 2017.

   [I-D.thubert-roll-unaware-leaves]
              Thubert, P., "Routing for RPL Leaves", draft-thubert-roll-
              unaware-leaves-05 (work in progress), May 2018.






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   [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>.

   [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>.




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   [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>.

   [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>.

11.4.  External Informative References

   [IEEEstd802154]
              IEEE, "IEEE Standard for Low-Rate Wireless Networks",
              IEEE Standard 802.15.4, DOI 10.1109/IEEE
              P802.15.4-REVd/D01, June 2017,
              <http://ieeexplore.ieee.org/document/7460875/>.



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   [Perlman83]
              Perlman, R., "Fault-Tolerant Broadcast of Routing
              Information", North-Holland Computer Networks 7: 395-405,
              1983, <http://www.cs.illinois.edu/~pbg/courses/cs598fa09/
              readings/p83.pdf>.

Appendix A.  Applicability and Requirements Served (Not Normative)

   This specification extends 6LoWPAN ND to provide a sequence number to
   the registration and serves 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 a companion document "IPv6 Backbone Router"
   [I-D.ietf-6lo-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 intself 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

   "6TiSCH architecture" [I-D.ietf-6tisch-architecture] describes how a
   6LoWPAN ND host using the Timeslotted Channel Hopping (TSCH) mode of
   IEEE Std. 802.15.4 [IEEEstd802154] can connect to the Internet via a
   RPL mesh network.  Doing so requires additions to the 6LoWPAN ND



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   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.

   The term LLN is used loosely in this document, and 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 including proxy-ND operations over a Backbone Link.  This
   satisfies the the requirements expressed in Appendix B.4.

   This specification is extended by "Address Protected Neighbor
   Discovery for Low-power and Lossy Networks" [I-D.ietf-6lo-ap-nd] to
   provide a solution to some of the security-related requirements
   expressed in Appendix B.5.

   "Efficiency aware IPv6 Neighbor Discovery Optimizations"
   [I-D.chakrabarti-nordmark-6man-efficient-nd] 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 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
   [I-D.ietf-mboned-ieee802-mcast-problems].  This serves the
   scalability requirements listed in Appendix B.6.

Appendix B.  Requirements (Not Normative)

   This section lists requirements that were discussed by the 6lo WG 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 6LR-a to 6LR-b,
   and 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



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   and restore reachability in a timely fashion, e.g., by using some
   signaling upon the detection of the movement, or using a keep-alive
   mechanism with a period that is consistent with the application
   needs.

   Req1.1: 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.

   Req1.2: For that purpose, the protocol MUST enable differentiating
   between multiple registrations from one 6LoWPAN Node and
   registrations from different 6LoWPAN Nodes claiming the same address.

   Req1.3: Stale states MUST be cleaned up in 6LRs.

   Req1.4: A 6LoWPAN Node 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 (SDO) on the basis of the expected network
   characteristics.  It is required that a 6LN attached via ND to a 6LR
   indicates whether it participates in the selected routing protocol to
   obtain reachability via the 6LR, or whether it expects the 6LR to
   manage its reachability.

   The specified updates enable other specifications to define new
   services such as Source Address Validation (SAVI) with
   [I-D.ietf-6lo-ap-nd], participation as an unaware leaf to a routing
   protocol such as the "Routing Protocol for Low Power and Lossy
   Networks" [RFC6550] (RPL) with [I-D.thubert-roll-unaware-leaves], and
   registration to a backbone routers performing proxy Neighbor
   Discovery in a Low-Power and Lossy Network (LLN) with
   [I-D.ietf-6lo-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 sub-tree), or
   both.



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   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.

   Related requirements are:

   Req2.1: 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.

   Req2.2: Considering RPL, the Address Registration Option 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.

   Req2.3: Multicast operations SHOULD be supported and optimized, for
   instance, using BIER or MPL.  Whether ND is appropriate for the
   registration to the Routing Registrar is to be defined, considering
   the additional burden of supporting the Multicast Listener Discovery
   Version 2 [RFC3810] (MLDv2) for IPv6.

B.3.  Requirements Related to the Variety of 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) [RFC6282] technique
   to other link types including ITU-T G.9959 [RFC7428], Master-Slave/
   Token-Passing [RFC8163], DECT Ultra Low Energy [RFC8105], Near Field
   Communication [I-D.ietf-6lo-nfc], IEEE Std. 802.11ah
   [I-D.delcarpio-6lo-wlanah], as well as Bluetooth(R) Low Energy
   [RFC7668], and Power Line Communication (PLC) [I-D.hou-6lo-plc]
   Networks.

   Related requirements are:

   Req3.1: 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.

   Req3.2: As part of this extension, a mechanism to compute a unique
   identifier should be provided, with the capability to form a Link-
   Local Address that SHOULD be unique at least within the LLN connected
   to a 6LBR discovered by ND in each node within the LLN.




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   Req3.3: The Address Registration Option used in the ND registration
   SHOULD be extended to carry the relevant forms of unique Identifier.

   Req3.4: The Neighbor Discovery should specify the formation of a
   site-local address that follows the security recommendations from
   [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 not capable to defend
   their own addresses.

   Related requirements are:

   Req4.1: The registration mechanism SHOULD enable a third party to
   proxy register an address on behalf of a 6LoWPAN node that may be
   sleeping or located deeper in an LLN mesh.

   Req4.2: 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.

   Req4.3: 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 then should 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



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   links.  The keys for the Layer-2 security are distributed by the JA/
   CT.  The JA/CT can be part of the LLN or be outside the LLN.  In both
   cases it is needed that packets are routed between JA/CT and the
   joining node.

   Related requirements are:

   Req5.1: 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 6LoWPAN
   Node for the role of 6LR.

   Req5.2: 6LoWPAN ND security mechanisms SHOULD provide a mechanism for
   the 6LR and the 6LBR to validate new registration of authorized
   nodes.  Joining of unauthorized nodes MUST be prevented.

   Req5.3: 6LoWPAN ND security mechanisms SHOULD NOT lead to 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 [IEEEstd802154] frame.

   Req5.4: Recurrent 6LoWPAN ND security operations MUST NOT be
   computationally intensive on the LoWPAN Node CPU.  When a Key hash
   calculation is employed, a mechanism lighter than SHA-1 SHOULD be
   used.

   Req5.5: The number of Keys that the 6LoWPAN Node needs to manipulate
   SHOULD be minimized.

   Req5.6: The 6LoWPAN ND security mechanisms SHOULD enable the
   variation of CCM [RFC3610] called CCM* for use at both Layer 2 and
   Layer 3, and SHOULD enable the reuse of security code that has to be
   present on the device for upper layer security such as TLS.
   Algorithm agility and support for large keys (e.g., 256-bit key
   sizes) is also desirable, following at Layer-3 the introduction of
   those capabilities at Layer-2.

   Req5.7: 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.

   Req5.8: Routing of packets should continue when links pass from the
   unsecured to the secured state.

   Req5.9: 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



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   initially, and, if not, determine the rightful owner and deny or
   clean up the registration that is duplicate.

B.6.  Requirements Related to Scalability

   Use cases from Automatic Meter Reading (AMR, collection tree
   operations) and Advanced Metering Infrastructure (AMI, bi-directional
   communication to the meters) indicate the needs 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:

   Req6.1: The registration mechanism SHOULD enable a single 6LBR to
   register multiple thousands of devices.

   Req6.2: The timing of the registration operation should allow for a
   large latency such as found in LLNs with ten to more hops.

B.7.  Requirements Related to Operations and Management

   Section 3.8 of "Architectural Principles of the Internet" [RFC1958]
   recommends to: "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 a dynamic configuration of LLN devices
   can also be constrained by the network and power limitation.

   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 registration, so the
   administrator can take actions such as adding a Backbone Link with
   additional 6LBRs and Routing Registrars to the network.

   Related requirements are:

   Req7.1: A management model SHOULD be provided that enables access to
   the 6LBR, monitor its usage vs. capacity, and alert in case of
   congestion.  It is recommended that the 6LBR be reachable over a non-
   LLN link.

   Req7.2: A management model SHOULD be provided that enables access to
   the 6LR and its capacity to host additional NCE.  This management
   model SHOULD avoid polling individual 6LRs in a way that could
   disrupt the operation of the LLN.





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   Req7.3: Information on successful and failed registration 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.

   Req7.4: In 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

                   I-drafts/RFCs addressing requirements

         +-------------+-----------------------------------------+
         | Requirement | Document                                |
         +-------------+-----------------------------------------+
         | Req1.1      | [I-D.ietf-6lo-backbone-router]          |
         |             |                                         |
         | Req1.2      | [RFC6775]                               |
         |             |                                         |
         | Req1.3      | [RFC6775]                               |
         |             |                                         |
         | Req1.4      | This RFC                                |
         |             |                                         |
         | Req2.1      | This RFC                                |
         |             |                                         |
         | Req2.2      | This RFC                                |
         |             |                                         |
         | Req2.3      |                                         |
         |             |                                         |
         | Req3.1      | Technology Dependent                    |
         |             |                                         |
         | Req3.2      | Technology Dependent                    |
         |             |                                         |
         | Req3.3      | Technology Dependent                    |
         |             |                                         |
         | Req3.4      | Technology Dependent                    |
         |             |                                         |
         | Req4.1      | This RFC                                |
         |             |                                         |
         | Req4.2      | This RFC                                |
         |             |                                         |
         | Req4.3      | [RFC6775]                               |
         |             |                                         |
         | Req5.1      |                                         |
         |             |                                         |
         | Req5.2      | [I-D.ietf-6lo-ap-nd]                    |
         |             |                                         |



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         | Req5.3      |                                         |
         |             |                                         |
         | Req5.4      |                                         |
         |             |                                         |
         | Req5.5      | [I-D.ietf-6lo-ap-nd]                    |
         |             |                                         |
         | Req5.6      | [I-D.struik-lwip-curve-representations] |
         |             |                                         |
         | Req5.7      | [I-D.ietf-6lo-ap-nd]                    |
         |             |                                         |
         | Req5.8      |                                         |
         |             |                                         |
         | Req5.9      | [I-D.ietf-6lo-ap-nd]                    |
         |             |                                         |
         | Req6.1      | This RFC                                |
         |             |                                         |
         | Req6.2      | This RFC                                |
         |             |                                         |
         | Req7.1      |                                         |
         |             |                                         |
         | Req7.2      |                                         |
         |             |                                         |
         | Req7.3      |                                         |
         |             |                                         |
         | Req7.4      |                                         |
         +-------------+-----------------------------------------+

                   Table 8: Work Addressing requirements

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




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   Samita Chakrabarti
   Verizon
   San Jose, CA
   United States of America

   Email: samitac.ietf@gmail.com


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

   Email: charliep@computer.org




































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