6lo                                                      P. Thubert, Ed.
Internet-Draft                                                     cisco
Updates: 6775 (if approved)                                  E. Nordmark
Intended status: Standards Track
Expires: March 24, 2018                                   S. Chakrabarti
                                                      September 20, 2017

                        An Update to 6LoWPAN ND


   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 backbone
   routers performing 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.

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   This Internet-Draft will expire on March 24, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Applicability of Address Registration Options . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Updating RFC 6775 . . . . . . . . . . . . . . . . . . . . . .   5   6
     4.1.  Extended Address Registration Option (EARO (EARO) . . . . . . .   7
     4.2.  Transaction ID  . . . . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Comparing TID values  . . . . . . . . . . . . . . . .   8
     4.3.  Owner Unique ID . . . . . . . . . . . . . . . . . . . . .   9
     4.4.  Extended Duplicate Address Messages . . . . . . . . . . .  10
     4.5.  Registering the Target Address  . . . . . . . . . . . . .  10
     4.6.  Link-Local Addresses and Registration . . . . . . . . . .  11
     4.7.  Maintaining the Registration States . . . . . . . . . . .  13
   5.  Detecting Enhanced ARO Capability Support . . . . . . . . . .  14
   6.  Extended ND Options And Messages  . . . . . . . . . . . . . .  15
     6.1.  Enhanced Address Registration Option (EARO) . . . . . . .  15
     6.2.  Extended Duplicate Address Message Formats  . . . . . . .  18
     6.3.  New 6LoWPAN Capability Bits in the Capability Indication
           Option  . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .  19
     7.1.  Discovering the capabilities of an ND peer  . . . . . . .  19
       7.1.1.  Using the E "E" Flag in the 6CIO Option . . . . . . . . .  19
       7.1.2.  Using the T "T" Flag in the EARO  . . . . . . . . . . . .  20
     7.2.  Legacy 6LoWPAN Node . . . . . . . . . . . . . . . . . . .  21
     7.3.  Legacy 6LoWPAN Router . . . . . . . . . . . . . . . . . .  21
     7.4.  Legacy 6LoWPAN Border Router  . . . . . . . . . . . . . .  22
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  23
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     10.1.  ARO Flags  . . . . . . . . . . . . . . . . . . . . . . .  24
     10.2.  ICMP Codes . . . . . . . . . . . . . . . . . . . . . . .  24
     10.3.  New ARO Status values  . . . . . . . . . . . . . . . . .  25
     10.4.  New 6LoWPAN capability Bits  . . . . . . . . . . . . . .  26
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  26
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     12.2.  Informative References . . . . . . . . . . . . . . . . .  27
     12.3.  External Informative References  . . . . . . . . . . . .  30
   Appendix A.  Applicability and Requirements Served  . . . . . . .  30
   Appendix B.  Requirements . . . . . . . . . . . . . . . . . . . .  31
     B.1.  Requirements Related to Mobility  . . . . . . . . . . . .  32
     B.2.  Requirements Related to Routing Protocols . . . . . . . .  32
     B.3.  Requirements Related to the Variety of Low-Power Link
           types . . . . . . . . . . . . . . . . . . . . . . . . . .  33
     B.4.  Requirements Related to Proxy Operations  . . . . . . . .  34
     B.5.  Requirements Related to Security  . . . . . . . . . . . .  34
     B.6.  Requirements Related to Scalability . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   The scope of this draft is an IPv6 Low Power Networks including star
   and mesh topologies.  This specification modifies and extends the
   behavior and protocol elements of "Neighbor Discovery Optimization
   for IPv6 over Low-Power Wireless Personal Area Networks" (6LoWPAN ND)
   [RFC6775]  to enable additional capabilities such as:

   o  Support for indicating mobility vs retry (T-bit)

   o  Ease up requirement of registration for link-local addresses

   o  Enhancement to Address Registration Option (ARO)

   o  Permitting registration of target address

   o  Clarification of support of privacy and temporary addresses

   The applicability of 6LoWPAN ND registration is discussed in
   Section 2, and new extensions and updates to RFC 6775 are presented
   in Section 4.  Considerations on Backward Compatibility, Security and
   Privacy are also elaborated upon in Section 7, Section 8 and in
   Section 9, respectively.

2.  Applicability of Address Registration Options

   The original purpose of the Address Registration Option (ARO) in the
   original 6LoWPAN ND specification is to facilitate duplicate address
   detection (DAD) for hosts as well as populate Neighbor Cache Entries
   (NCE) [RFC4861] in the routers.  This reduces the reliance on
   multicast operations, which are often as intrusive as broadcast, in
   IPv6 ND operations.

   With this specification, a registration can fail or become useless
   for reasons other than address duplication.  Examples include: the
   router having run out of space; a registration bearing a stale
   sequence number perhaps denoting a movement of the host after the
   registration was placed; a host misbehaving and attempting to
   register an invalid address such as the unspecified address
   [RFC4291]; or a host using an address which 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.
   However, the ability to return errors to address registrations is not
   intended to be used to restrict the ability of hosts to form and use
   addresses, as recommended in "Host Address Availability
   Recommendations" [RFC7934].

   In particular, the freedom to form and register addresses is needed
   for enhanced privacy; each host may register a multiplicity of
   address using mechanisms such as "Privacy Extensions for Stateless
   Address Autoconfiguration (SLAAC) in IPv6" [RFC4941].

   In the classical IPv6 ND [RFC4861], a router must have enough storage
   to hold neighbor cache entries for all the addresses to which it may
   forward.  A router using 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.  For this reason, the number of registrations
   supported by a 6LoWPAN Router (6LR) or 6LoWPAN Border Router (6LBR)
   must be clearly documented.

   A network administrator should deploy adapted 6LR/6LBRs to support
   the number and type of devices in his network, based on the number of
   IPv6 addresses that those devices require and their renewal rate and

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Readers are expected to be familiar with all the terms and concepts
   that are discussed in

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

   o  "Multi-link Subnet Support in IPv6"

   as well as the following terminology:

   Backbone Link  An IPv6 transit link that interconnects two or more
         Backbone Routers.  It is expected to be of a relatively high
         speed compared to the LLN in order to support the trafic that
         is required to federate multiple segments of the potentially
         large LLN into a single IPv6 subnet.  Also referred to as a to
         as a Backbone, a LLN Backbone, and a Backbone Network.

   Backbone Router  A logical network function in an IPv6 router that
         federates a LLN over a Backbone Link.  In order to do so, the
         Backbone Router (6BBR) proxies the 6LoWPAN ND operations
         detailed in the document onto the matching operations that run
         over the backbone, typically classical IPv6 ND.  Note that 6BBR
         is a logical function, just like 6LR and 6LBR, and that a same
         physical router may operate all three.

   Extended LLN  The aggregation of multiple LLNs as defined in RFC 4919
         [RFC4919], interconnected by a Backbone Link via Backbone
         Routers, and forming a single IPv6 MultiLink Subnet.

   Registration  The process during which a wireless Node registers its
         address(es) with the Border Router so the 6BBR can serve as
         proxy for ND operations over the Backbone.

   Binding  The association between an IP address with a MAC address, a
         port and/or other information about the node that owns the IP

   Registered Node  The node for which the registration is performed,
         and which owns the fields in the EARO option.

   Registering Node  The node that performs the registration to the
         6BBR, which may proxy for the registered node.

   Registered Address  An address owned by the Registered Node node that
         was or is being registered.

   legacy and original vs. updated  In the context of this
         specification, the terms "legacy" and "original" relate to the
         support of the RFC 6775 by a 6LN, a 6LR or a 6LBR, whereas the
         term "updated" refers to the support of this specification.

   classical  In the context of this specification, the term "classical"
         relates to the support of the IPv6 Neighbor Discovery (IPv6 ND)
         protocol as specified in RFC 4861 and RFC 4862.  This
         specification does not deprecate the classical IPv6 ND

4.  Updating RFC 6775

   This specification introduces the Extended Address Registration
   Option (EARO) based on the ARO as defined in RFC 6775 [RFC6775]; in
   particular a "T" flag is added that must MUST be set is NS messages when
   this specification is used, and echoed in NA messages to confirm that
   the protocol is supported.

   Support for this specification can thus be inferred from the presence
   of the Extended ARO ("T" flag set) in 6LoWPAN ND messages.

   The extensions to the ARO option are reported to the Duplicate
   Address Request (DAR) and Duplicate Address Confirmation (DAC)
   messages, so as to convey the additional information all the way to
   the 6LBR, and in turn the 6LBR may proxy the registration using
   classical ND over a backbone as illustrated in Figure 1.

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

                     Figure 1: (Re-)Registration Flow

   In order to support various types of link layers, this specification
   to allow multiple registrations, including for privacy / temporary
   addresses, and provides new mechanisms to help clean up stale
   registration states as soon as possible.

   A Registering Node that supports this specification SHOULD prefer registering to a 6LR that is found
   to support this specification, as discussed in Section 7.1, over a
   legacy one.

4.1.  Extended Address Registration Option (EARO

   This specification extends the ARO option that is used for the
   process of address registration. (EARO)

   The new ARO is referred to as Extended ARO (EARO), (EARO) deprecates the ARO and it is backward compatible
   with the ARO. it.  More details on backward compatibility can be found in
   Section 7.

   The semantics of the ARO are modified as follows:

   o  The address that is being registered with a Neighbor Solicitation
      (NS) with an EARO is now the Target Address, as opposed to the
      Source Address as specified in RFC 6775 [RFC6775] (see
      Section 4.5).  This change enables a 6LBR to use one of its
      addresses as source to the proxy-registration of an address that
      belongs to a LLN Node to a 6BBR.  This also limits the use of an
      address as source address before it is registered and the
      associated DAD process is complete.

   o  The Unique ID in the EARO Option is no longer required to be a MAC
      address (see Section 4.3).  This enables in particular the use of
      a Provable Temporary UID (PT-UID) as opposed to burn-in MAC
      address; the PT-UID provides an anchor trusted by the 6LR and 6LBR
      to protect the state associated to the node.

   o  The specification introduces a Transaction ID (TID) field in the
      EARO (see Section 4.2).  The TID MUST be provided by a node that
      supports this specification and a new "T" flag MUST be set to
      indicate so.

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

4.2.  Transaction ID

   The Transaction ID (TID) is a sequence number that is incremented
   with each re-registration.  The TID is used to detect the freshness
   of the registration request and useful to detect one single
   registration by multiple 6LOWPAN border routers (e.g., 6LBRs and
   6BBRs) supporting the same 6LOWPAN.  The TID may also be used by the
   network to track the sequence of movements of a node in order to
   route to the current (freshest known) location of a moving node.

   When a Registered Node is registered with multiple BBRs in parallel,
   the same TID SHOULD be used, to enable the 6BBRs to determine that
   the registrations are the same, and distinguish that situation from a

4.2.1.  Comparing TID values

   The TID is a sequence counter and its operation is the exact match of
   the path sequence specified in RPL, the IPv6 Routing Protocol for
   Low-Power and Lossy Networks [RFC6550] specification.

   In order to keep this document self-contained and yet compatible, the
   text below is an exact copy from section 7.2.  "Sequence Counter
   Operation" of [RFC6550].

   A TID is deemed to be fresher than another when its value is greater
   per 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 detected 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.

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

       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

           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

           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
       consider the comparison as if it has evaluated in such a way so
       as to give precedence to the sequence number that has most
       recently been observed to increment.  Failing this, the node
       should consider the comparison as if it has evaluated in such a
       way so as to minimize the resulting changes to its own state.

4.3.  Owner Unique ID

   The Owner Unique ID (OUID) enables a duplicate address registration
   to be distinguished from a double registration or a movement.  An ND
   message from the 6BBR over the Backbone that is proxied on behalf of
   a Registered Node must carry the most recent EARO option seen for
   that node.  A NS/NA with an EARO and a NS/NA without a EARO thus
   represent different nodes; if they relate to a same target then an
   address duplication is likely.

   With RFC 6775, the Owner Unique ID carries an EUI-64 burn-in address,
   which implies that duplicate EUI-64 addresses are avoided.  With this
   specification, the Owner Unique ID is allowed to be extended to
   different types of identifier, as long as the type is clearly
   indicated.  For instance, the type can be a cryptographic string and
   used to prove the ownership of the registration as discussed in
   "Address Protected Neighbor Discovery for Low-power and Lossy
   Networks" [I-D.ietf-6lo-ap-nd].

   In any fashion, it is recommended that the node stores the unique Id
   or the keys used to generate that ID in persistent memory.
   Otherwise, it will be prevented to re-register a same address after a
   reboot that would cause a loss of memory until the 6LBR times out the

4.4.  Extended Duplicate Address Messages

   In order to map the new EARO content in the DAR/DAC messages, a new
   TID field is added to the Extended DAR (EDAR) and the Extended DAC
   (EDAC) messages as a replacement to a Reserved field, and an odd
   value of the ICMP Code indicates support for the TID, to transport
   the "T" flag.

   In order to prepare for new extensions, and though no option had been
   earlier defined for the Duplicate Address messages, implementations
   SHOULD expect ND options after the main body, and SHOULD ignore them.

   As for the EARO, the Extended Duplicate Address messages are backward
   compatible with the original versions, and remarks concerning
   backwards compatibility between the 6LN and the 6LR apply similarly
   between a 6LR and a 6LBR.

4.5.  Registering the Target Address

   The Registering Node is the node that performs the registration to
   the 6BBR.  As inherited from RFC 6775, 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 a LLN
   mesh.  In that case, if the Registered Node is reachable from the
   6BBR over a Mesh-Under mesh, the Registering Node indicates the MAC
   Address of the Registered Node as 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 6BBR 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.

   The reason for this change is to enable proxy-registrations on behalf
   of other nodes, for instance to enable a RPL root to register
   addresses on behalf of other LLN nodes, as discussed in Appendix B.4.
   In that case, the Registering Node MUST indicate its own address as
   source of the ND message and its MAC address in the Source Link-Layer
   Address Option (SLLAO), since it still expects to receive and route
   the packets.  Since the Registered Address belongs to the Registered
   Node, that address is indicated in the Target Address field of the NS

   With this convention, a TLLA option indicates the link-layer address
   of the 6LN that owns the address, whereas the SLLA Option in a NS
   message indicates that of the Registering Node, which can be the
   owner device, or a proxy.

   The Registering Node is reachable from the 6LR, and is also the one
   expecting packets for the 6LN.  Therefore, it MUST place its own Link
   Layer Address in the SLLA Option that MUST always be placed in a
   registration NS(EARO) message.  This maintains compatibility with the
   original 6LoWPAN ND [RFC6775].

4.6.  Link-Local Addresses and Registration

   Considering that LLN nodes are often not wired and may move, there is
   no guarantee that a Link-Local address stays unique between a
   potentially variable and unbounded set of neighboring nodes.

   Compared to RFC 6775, this specification only requires that a Link-
   Local address is unique from the perspective of the nodes that use it
   to communicate (e.g. the 6LN and the 6LR in an NS/NA exchange).  This
   simplifies the DAD process for Link-Local addresses, and there is no
   exchange of Duplicate Address messages between the 6LR and a 6LBR for
   Link-Local addresses.

   According to RFC 6775, a 6LoWPAN Node (6LN) uses the an address being
   registered as the source of the registration message.  This generates
   complexities in the 6LR to be able to cope with a potential
   duplication, in particular for global addresses.

   To simplify this, a 6LN and a 6LR that conform this specification
   MUST always use Link-Local addresses as source and destination
   addresses for the registration NS/NA exchange.  As a result, the
   registration is globally faster, and some of the complexity is

   In more details:

   An exchange between two nodes using Link-Local addresses implies that
   they are reachable over one hop and that at least one of the 2 nodes
   acts as a 6LR.  A node MUST register a Link-Local address to a 6LR in
   order to obtain reachability from that 6LR beyond the current
   exchange, 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 an address previously registered to
   this 6LR by another 6LN, then, from the standpoint of this 6LR, this
   Link-Local address is unique and the registration is acceptable.
   Conversely, it may possibly happen that two different 6LRs expose the
   same Link-Local address but different link-layer addresses.  In that
   case, a 6LN may only interact with one of the 6LRs so as to avoid
   confusion in the 6LN neighbor cache.

   The DAD process between the 6LR and a 6LBR, which is based on an
   exchange of Duplicate Address messages, does not need to take place
   for Link-Local addresses.

   It is desired that a 6LR does not need to modify its state associated
   to the Source Address of an NS(EARO) message.  For that reason, when
   possible, it is RECOMMENDED to use an address that is already
   registered with a 6LR

   When registering to a 6LR that conforms this specification, a node
   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 already
   registered, or the address that is being registered.

   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
   RECOMMENDED to use a Link-Local address that is (expected to be)
   globally unique, e.g.  derived from a burn-in MAC address.  An EARO
   option in the response NA indicates that the 6LR supports this

   Since there is no Duplicate Address exchange 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 MUST be placed in the NS(EARO) message as
   required in RFC 6775 [RFC6775].

   A node needs to register its IPv6 Global Unicast IPv6 Addresses
   (GUAs) to a 6LR in order to establish global reachability for these
   addresses via that 6LR.  When registering with a 6LR that conforms
   this specification, a Registering Node does not use its GUA as Source
   Address, in contrast to a node that complies to RFC 6775 [RFC6775].
   For non-Link-Local addresses, the Duplicate Address exchange MUST
   conform to RFC 6775, 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.

4.7.  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, which, as discussed in Section 4.6, 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 in a 6LR. 6LBRs and 6BBRs may store
   additional registration information in more complex data structures
   and use protocols that are out of scope of this document to keep them
   synchonized when they are distributed.

   When its Neighbor Cache is full, a 6LR cannot accept a new
   registration.  In that situation, the EARO is returned in a NA
   message with a Status of 2, and the Registering Node may attempt to
   register to another 6LR.

   Conversely the registry in the 6LBR may be saturated, in which case
   the LBR cannot guarantee that a new address is effectively not a
   duplicate.  In that case, the 6LBR replies to a EDAR message with a
   EDAC message that carries a Status code 9 indicating "6LBR Registry
   saturated", and the address stays in TENTATIVE state.  Note: this
   code is used by 6LBRs instead of Status 2 when responding to a
   Duplicate Address message exchange and passed on to the Registering
   Node by the 6LR.  There is no point for the node to retry this
   registration immediately via another 6LR, since the problem is global
   to the network.  The node may either abandon that address, deregister
   other addresses first to make room, or keep the address in TENTATIVE
   state and retry later.

   A node renews an existing registration by repeatedly sending NS(EARO)
   messages for the Registered Address.  In order to refresh the
   registration state in the 6LBR, these registrations MUST be reported
   to the 6LBR.

   A node that ceases to use an address SHOULD attempt to deregister
   that address from all the 6LRs to which it has registered the
   address, which is achieved using an NS(EARO) message with a
   Registration Lifetime of 0.

   A node that moves away from a particular 6LR SHOULD attempt to
   deregister all of its addresses registered to that 6LR and register
   to a new 6LR with an incremented TID.  When/if the node shows up
   elsewhere, an asynchronous NA(EARO) or EDAC message with a status of
   3 "Moved" SHOULD be used to clean up the state in the previous
   location.  For instance, the "Moved" status can be used by a 6BBR in
   a NA(EARO) message to indicate that the ownership of the proxy state
   on the Backbone was transferred to another 6BBR, as the consequence
   of a movement of the device.  The receiver of the message SHOULD
   propagate the status down the chain towards the Registered node and
   clean up its state.

   Upon receiving a NS(EARO) message with a Registration Lifetime of 0
   and determining that this EARO is the freshest for a given NCE (see
   Section 4.2), 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, or an alternate protocol,
   indicating the null Registration Lifetime and the latest TID that
   this 6LR is aware of.

   Upon the Extended DAR message, the 6LBR evaluates if this is the
   freshest TID it has received for that particular registry entry.  If
   it is, then the entry is scheduled to be removed, and the EDAR is
   answered with a EDAC message bearing a Status of 0 "Success".  If it
   is not the freshest, then a Status 3 "Moved" is returned instead, and
   the existing entry is conserved.

   Upon timing out a registration, a 6LR removes silently its binding
   cache entry, and a 6LBR schedules its entry to be removed.

   When an address is scheduled to be removed, the 6LBR SHOULD keep its
   entry in a DELAY state for a configurable period of time, so as to
   protect a mobile node that deregistered from one 6LR and did not
   register yet to a new one, or the new registration did not reach yet
   the 6LBR due to propagation delays in the network.  Once the DELAY
   time is passed, the 6LBR removes silently its entry.

5.  Detecting Enhanced ARO Capability Support

   The "Generic Header Compression for IPv6 over 6LoWPANs" [RFC7400]
   introduces the 6LoWPAN Capability Indication Option (6CIO) to
   indicate a node's capabilities to its peers.  This specification
   extends the format defined in RFC 7400 to signal the support for
   EARO, as well as the node's capability to act as a 6LR, 6LBR and

   With RFC 7400, the 6CIO is typically sent in a Router Solicitation
   (RS) message.  When used to signal the capabilities above per this
   specification, the 6CIO is typically present in Router Advertisement
   (RA) messages but can also be present in RS, Neighbor Solicitation
   (NS) and Neighbor Advertisement (NA) messages.

6.  Extended ND Options And Messages

   This specification does not introduce new options, but it modifies
   existing ones and updates the associated behaviors as specified in
   the following subsections.

6.1.  Enhanced Address Registration Option (EARO)

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

   The Enhanced Address Registration Option (EARO) is intended to be
   used as a replacement to the ARO option within Neighbor Discovery NS
   and NA messages between a 6LN and its 6LR.  Conversely, the Extended
   Duplicate Address messages, EDAR and EDAC, are to be used in
   replacement of the DAR and DAC messages so as to transport the new
   information between 6LRs and 6LBRs across LLNs meshes such as 6TiSCH

   An NS message with an EARO option is a registration if and only if it
   also carries an SLLAO option.  The EARO option also used in NS and NA
   messages between Backbone Routers over the Backbone link to sort out
   the distributed registration state; in that case, it does not carry
   the SLLAO option and is not confused with a registration.

   When using the EARO option, the address being registered is found in
   the Target Address field of the NS and NA messages.  This differs
   from 6LoWPAN ND RFC 6775 [RFC6775] which specifies that the address
   being registered is the source of the NS.

   The EARO extends the ARO and is recognized by the "T" flag set.  The
   format of the EARO option is as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |     Type      |   Length = 2  |    Status     |   Reserved    |
     |   Reserved  |T|     TID       |     Registration Lifetime     |
     |                                                               |
     +         Owner Unique ID   (EUI-64 or equivalent)              +
     |                                                               |

                              Figure 2: EARO

   Option Fields

   Type:           33

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

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

   | Value | Description                                               |
   |  0..2 | See RFC 6775 [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" should be used.                |
   |       |                                                           |
   |   3   | Moved: The registration fails because it is not the       |
   |       | freshest.  This Status indicates that the registration is |
   |       | rejected because another more recent registration was     |
   |       | done, as indicated by a same OUI 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 OUI collision.      |
   |       |                                                           |
   |   4   | Removed: The binding state was removed. This may be       |
   |       | placed in an asynchronous NS(ARO) message, or as the      |
   |       | rejection of a proxy registration to a Backbone Router    |
   |       |                                                           |
   |   5   | Validation Requested: The Registering Node is challenged  |
   |       | for owning the Registered Address or for being an         |
   |       | acceptable proxy for the registration.  This Status is    |
   |       | expected in asynchronous messages from a registrar (6LR,  |
   |       | 6LBR, 6BBR) to indicate that the registration state is    |
   |       | removed, for instance due to a movement of the device.    |
   |       |                                                           |
   |   6   | Duplicate Source Address: The address used as source of   |
   |       | the NS(ARO) conflicts with an existing registration.      |
   |       |                                                           |
   |   7   | Invalid Source Address: The address used as source of the |
   |       | NS(ARO) is not a Link-Local address as prescribed by this |
   |       | document.                                                 |
   |       |                                                           |
   |   8   | Registered Address topologically incorrect: The address   |
   |       | being registered is not usable on this link, e.g. it is   |
   |       | not topologically correct                                 |
   |       |                                                           |
   |   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    |
   |       | 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

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

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

   TID:            1-byte integer; a transaction id that is maintained
                   by the node and incremented with each transaction.
                   The node SHOULD maintain the TID in a persistent

   Registration Lifetime:  16-bit integer; expressed in minutes.  0
                   means that the registration has ended and the
                   associated state should be removed.

   Owner Unique Identifier (OUI):  A globally unique identifier for the
                   node associated.  This can be the EUI-64 derived IID
                   of an interface, or some provable ID obtained

6.2.  Extended Duplicate Address Message Formats

   The Duplicate Address Request (DAR) and the Duplicate Address
   Confirmation (DAC) messages are defined in section 4.4. of [RFC6775].
   Those messages follow a common base format, which enables information
   from the ARO to be transported over multiple hops.

   The Duplicate Address Messages are extended to adapt to the Extended
   ARO format, as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |     Type      |     Code      |          Checksum             |
      |    Status     |     TID       |     Registration Lifetime     |
      |                                                               |
      +          Owner Unique ID   (EUI-64 or equivalent)             +
      |                                                               |
      |                                                               |
      +                                                               +
      |                                                               |
      +                       Registered Address                      +
      |                                                               |
      +                                                               +
      |                                                               |

                Figure 3: Duplicate Address Messages Format

   Modified Message Fields

   Code:           The ICMP Code as defined in [RFC4443].  The ICMP Code
                   MUST be set to 1 with this specification.  An odd
                   value of the ICMP Code indicates that the TID field
                   is present and obeys this specification.

   TID:            1-byte integer; same definition and processing as the
                   TID in the EARO option as defined in Section 6.1.

   Owner Unique Identifier (OUI):  8 bytes; same definition and
                   processing as the OUI in the EARO option as defined
                   in Section 6.1.

6.3.  New 6LoWPAN Capability Bits in the Capability Indication Option

   This specification defines a number of capability bits in the 6CIO
   that was introduced by RFC 7400 for use in IPv6 ND RA messages.

   Routers that support this specification SHOULD set the "E" flag and
   6LN SHOULD favor 6LR routers that support this specification over
   those that do not.  Routers that are capable of acting as 6LR, 6LBR
   and 6BBR SHOULD set the "L", "B" and "P" flags, respectively.  In
   particular, the function 6LR is usually collocated with that of 6LBR.

   Those flags are not mutually exclusive and if a router is capable of
   running multiple functions, it SHOULD set all the related flags.

       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        |L|B|P|E|G|
      |                         Reserved                              |

           Figure 4: New capability Bits L, B, P, E in the 6CIO

   Option Fields

   Type:  36

   L: Node is a 6LR, it can take registrations.

   B: Node is a 6LBR.

   P: Node is a 6BBR, proxying for nodes on this link.

   E: This specification is supported and applied.

7.  Backward Compatibility

7.1.  Discovering the capabilities of an ND peer

7.1.1.  Using the E "E" Flag in the 6CIO Option

   If the 6CIO is used in an ND message and the sending node supports
   this specification, then the "E" Flag MUST be set.

   A router that supports this specification SHOULD indicate that with a
   6CIO Option, but this might not be practical if the link-layer MTU is
   too small.

   If the Registering Node (RN) receives a CIO in a Router Advertisement
   message, then the setting of the "E" Flag indicates whether or not
   this specification is supported.  RN SHOULD favor a router that
   supports this specification over those that do not.

7.1.2.  Using the T "T" Flag in the EARO

   One alternate way for a 6LN to discover the router's capabilities to
   first register a Link Local address, placing the same address in the
   Source and Target Address fields of the NS message, and setting the
   "T" Flag.  The node may for instance register an address that is
   based on EUI-64.  For such address, DAD is not required and using the
   SLLAO option in the NS is actually more consistent with existing ND
   specifications such as the "Optimistic Duplicate Address Detection
   (DAD) for IPv6" [RFC4429].

   Once that first registration is complete, the node knows from the
   setting of the "T" Flag in the response whether the router supports
   this specification.  If support is verified, the node may register
   other addresses that it owns, or proxy-register addresses on behalf
   some another node, indicating those addresses being registered in the
   Target Address field of the NS messages, while using one of its own
   previously registered addresses as source.

   A node that supports this specification MUST always use an EARO as a
   replacement to an ARO in its registration to a router.  This is
   harmless since the "T" flag and TID field are reserved in RFC 6775
   are ignored by a legacy router.  A router that supports this
   specification answers an ARO with an ARO and answers an EARO with an

   This specification changes the behavior of the peers in a
   registration flows.  To enable backward compatibility, a 6LB that
   registers to a 6LR that is not known to support this specification
   MUST behave in a manner that is compatible with RFC 6775.  A 6LN can
   achieve that by sending a NS(EARO) message with a Link-Local Address
   used as both Source and Target Address, as described in Section 4.6.
   Once the 6LR is known to support this specification, the 6LN MUST
   obey this specification.

7.2.  Legacy 6LoWPAN Node

   A legacy 6LN will use the Registered Address as source and will not
   use an EARO option.  An updated 6LR MUST accept that registration if
   it is valid per RFC 6775, and it MUST manage the binding cache
   accordingly.  The updated 6LR MUST then use the original Duplicate
   Address messages as specified in RFC 6775 to indicate to the 6LBR
   that the TID is not present in the messages.

   The main difference with RFC 6775 is that Duplicate Address exchange
   for DAD is avoided for Link-Local addresses.  In any case, the 6LR
   SHOULD use an EARO in the reply, and may use any of the Status codes
   defined in this specification.

7.3.  Legacy 6LoWPAN Router

   The first registration by an updated 6LN MUST be for a Link-Local
   address, using that Link-Local address as source.  A legacy 6LR will
   not make a difference and accept -or reject- that registration as if
   the 6LN was a legacy node.

   An updated 6LN will always use an EARO option in the registration NS
   message, whereas a legacy 6LR will always reply with an ARO option in
   the NA message.  So from that first registration, the updated 6LN can
   figure whether the 6LR supports this specification or not.

   After detecting a legacy 6LR, an updated 6LN may attempt to find an
   alternate 6LR that is updated.  In order to be backward compatible,
   after detecting that a 6LR is legacy, the 6LN MUST adhere to RFC 6775
   in future protocol exchanges with that 6LR, and source the packet
   with the Registered Address.

   Note that the updated 6LN SHOULD use an EARO in the request
   regardless of the type of 6LR, legacy or updated, which implies that
   the 'T' "T" flag is set.

   If an updated 6LN moves from an updated 6LR to a legacy 6LR, the
   legacy 6LR will send a legacy DAR message, which can not be compared
   with an updated one for freshness.

   Allowing legacy DAR messages to replace a state established by the
   updated protocol in the 6LBR would be an attack vector and that
   cannot be the default behavior.

   But if legacy and updated 6LRs coexist temporarily in a network, then
   it makes sense for an administrator to install a policy that allows
   so, and the capability to install such a policy should be
   configurable in a 6LBR though it is out of scope for this document.

7.4.  Legacy 6LoWPAN Border Router

   With this specification, the Duplicate Address messages are extended
   to transport the EARO information.  Similarly to the NS/NA exchange,
   updated 6LBR devices always use the Extended Duplicate Address
   messages and all the associated behavior so they can amlways be
   differentiated from legacy ones.

   Note that a legacy 6LBR will accept and process an EDAR message as if
   it was an original one, so the original support of DAD is preserved.

8.  Security Considerations

   This specification extends RFC 6775 [RFC6775], and the security
   section of that draft also applies to this as well.  In particular,
   it is expected that the link layer is sufficiently protected to
   prevent a rogue access, either by means of physical or IP security on
   the Backbone Link and link layer cryptography on the LLN.

   This specification also expects that the LLN MAC provides secure
   unicast to/from the Backbone Router and secure Broadcast from the
   Backbone Router in a way that prevents tempering with or replaying
   the RA messages.

   This specification recommends to using privacy techniques (see
   Section 9, and protection against address theft such as provided by
   "Address Protected Neighbor Discovery for Low-power and Lossy
   Networks" [I-D.ietf-6lo-ap-nd], which guarantees the ownership of the
   Registered Address using a cryptographic OUID.

   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 registration, which effectively denies the
   requesting a node the capability to use a new address.  In order to
   alleviate those concerns, Section 4.7 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 deregister
      that address from all the 6LRs to which it is registered.  The
      flow is propagated to the 6LBR when needed, and a sequence number
      is used to make sure that only the freshest command is acted upon.

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

   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, as
      identified at least by MAC address and preferably by security
      credentials.  When that maximum is reached, the router should use
      a Least-Recently-Used (LRU) logic so as to clean up the addresses
      that were not used for the longest time, keeping at least one
      Link-Local address, and attempting to keep one or more stable
      addresses if such can be recognized, e.g. from the way the IID is
      formed or because they are used over a much longer time span than
      other (privacy, shorter-lived) addresses.  The address lifetimes
      SHOULD be individually configurable.

   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 a LLN is a more capable node
      then the average 6LR, but in a network condition where it may
      become saturated, a particular deployment SHOULD distribute the
      6LBR functionality, for instance by leveraging a high speed
      Backbone and Backbone Routers to aggregate multiple LLNs into a
      larger subnet.

   The LLN nodes depend on the 6LBR and the 6BBR for their operation.  A
   trust model must be put in place to ensure that the right 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.

9.  Privacy Considerations

   As indicated in section Section 2, this protocol does not aim at
   limiting the number of IPv6 addresses that a device can form.  A host
   should be able to form and register any address that is topologically
   correct in the subnet(s) advertised by the 6LR/6LBR.

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

   "Privacy Considerations for IPv6 Adaptation-Layer Mechanisms"
   [RFC8065] explains why privacy is important and how to form such
   addresses.  All implementations and deployment must consider the
   option of privacy addresses in their own environment.  Also future
   specifications involving 6LOWPAN Neighbor Discovery should consult
   "Recommendation on Stable IPv6 Interface Identifiers" [RFC8064] for
   default interface identifaction.

10.  IANA Considerations

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

10.1.  ARO Flags

   IANA is requested to create a new subregistry for "ARO Flags".  This
   specification defines 8 positions, bit 0 to bit 7, and assigns bit 7
   for the 'T' "T" flag in Section 6.1.  The policy is "IETF Review" or
   "IESG Approval" [RFC8126].  The initial content of the registry is as
   shown in Table 2.

     New subregistry for ARO Flags under the "Internet Control Message
             Protocol version 6 (ICMPv6) [RFC4443] Parameters"

                 | ARO Status | Description  | Document  |
                 |    0..6    | Unassigned   |           |
                 |     7      | 'T' "T" Flag     | RFC This  |

                          Table 2: new ARO Flags

10.2.  ICMP Codes

   IANA is requested to create a new entry in the ICMPv6 "Code" Fields
   subregistry of the Internet Control Message Protocol version 6
   (ICMPv6) Parameters for the ICMP codes related to the ICMP type 157
   and 158 Duplicate Address Request (shown in Table 3) and Confirmation
   (shown in Table 4), respectively, as follows:

                New entries for ICMP types 157 DAR message

               | Code | Name                 | Reference  |
               | 0    | Original DAR message | RFC 6775   |
               | 1    | Extended DAR message | RFC This   |

                      Table 3: new ICMPv6 Code Fields

                New entries for ICMP types 158 DAC message

               | Code | Name                 | Reference  |
               | 0    | Original DAC message | RFC 6775   |
               | 1    | Extended DAC message | RFC This   |

                      Table 4: new ICMPv6 Code Fields

10.3.  New ARO Status values

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

            Address Registration Option Status Values Registry

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

                      Table 5: New ARO Status values

10.4.  New 6LoWPAN capability Bits

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

   Subregistry for "6LoWPAN capability Bits" under the "Internet Control
              Message Protocol version 6 (ICMPv6) Parameters"

           | capability Bit | Description          | Document  |
           |       11       | 6LR  capable (L bit) | RFC This  |
           |       12       | 6LBR capable (B bit) | RFC This  |
           |       13       | 6BBR capable (P bit) | RFC This  |
           |       14       | EARO support (E bit) | RFC This  |

                   Table 6: New 6LoWPAN capability Bits

11.  Acknowledgments

   Kudos to Eric Levy-Abegnoli who designed the First Hop Security
   infrastructure upon which the first backbone router was implemented;
   many thanks to Charlie Perkins for his in-depth reviews and
   constructive suggestions, as well as to Sedat Gormus, Rahul Jadhav
   and Lorenzo Colitti for their various contributions and reviews.
   Also many thanks to Thomas Watteyne for his early implementation of a
   6LN that was instrumental to the early tests of the 6LR, 6LBR and
   Backbone Router.

12.  References

12.1.  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,

   [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,

   [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,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [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,

   [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,

   [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,

12.2.  Informative References

              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.

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

              Sarikaya, B., Thubert, P., and M. Sethi, "Address
              Protected Neighbor Discovery for Low-power and Lossy
              Networks", draft-ietf-6lo-ap-nd-02 (work in progress), May

              Thubert, P., "IPv6 Backbone Router", draft-ietf-6lo-
              backbone-router-04 (work in progress), July 2017.

              Choi, Y., Hong, Y., Youn, J., Kim, D., and J. Choi,
              "Transmission of IPv6 Packets over Near Field
              Communication", draft-ietf-6lo-nfc-07 (work in progress),
              June 2017.

              Thubert, P., "An Architecture for IPv6 over the TSCH mode
              of IEEE 802.15.4", draft-ietf-6tisch-architecture-12 (work
              in progress), August 2017.

              Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
              S. Aldrin, "Multicast using Bit Index Explicit
              Replication", draft-ietf-bier-architecture-08 (work in
              progress), September 2017.

              Thaler, D. and C. Huitema, "Multi-link Subnet Support in
              IPv6", draft-ietf-ipv6-multilink-subnets-00 (work in
              progress), July 2002.

              Popa, D. and J. Hui, "6LoPLC: Transmission of IPv6 Packets
              over IEEE 1901.2 Narrowband Powerline Communication
              Networks", draft-popa-6lo-6loplc-ipv6-over-
              ieee19012-networks-00 (work in progress), March 2014.

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,

   [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,

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, DOI 10.17487/RFC4919, August 2007,

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [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,

   [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,

   [RFC7428]  Brandt, A. and J. Buron, "Transmission of IPv6 Packets
              over ITU-T G.9959 Networks", RFC 7428,
              DOI 10.17487/RFC7428, February 2015,

   [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,

   [RFC7934]  Colitti, L., Cerf, V., Cheshire, S., and D. Schinazi,
              "Host Address Availability Recommendations", BCP 204,
              RFC 7934, DOI 10.17487/RFC7934, July 2016,

   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,

   [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>.

12.3.  External Informative References

              IEEE, "IEEE Standard for Low-Rate Wireless Networks",
              IEEE Standard 802.15.4, DOI 10.1109/IEEESTD.2016.7460875,

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

Appendix A.  Applicability and Requirements Served

   This specification extends 6LoWPAN ND to sequence the registration
   and serves the requirements expressed Appendix B.1 by enabling the
   mobility of devices from one LLN to the next based on the
   complementary work in the "IPv6 Backbone Router"
   [I-D.ietf-6lo-backbone-router] specification.

   In the context of the the TimeSlotted Channel Hopping (TSCH) mode of
   IEEE Std. 802.15.4 [IEEEstd802154], the "6TiSCH architecture"
   [I-D.ietf-6tisch-architecture] introduces how a 6LoWPAN ND host could
   connect to the Internet via a RPL mesh Network, but this requires
   additions to the 6LOWPAN ND protocol to support mobility and
   reachability in a secured and manageable environment.  This
   specification details the new operations that are required to
   implement the 6TiSCH architecture and serves the requirements listed
   in Appendix B.2.

   The term LLN is used loosely in this specification to cover multiple
   types of WLANs and WPANs, including Low-Power Wi-Fi, BLUETOOTH(R) 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 associate at
   Layer-3 with a 6BBR and register its IPv6 addresses to obtain routing
   services including proxy-ND operations over the Backbone, effectively
   providing a solution to the requirements expressed in Appendix B.4.

   "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 classical ND ([RFC4861],
   [RFC4862]) and plague the wireless medium.  This serves scalability
   requirements listed in Appendix B.6.

Appendix B.  Requirements

   This section lists requirements that were discussed at 6lo for an
   update to 6LoWPAN ND.  This specification meets most of them, but
   those listed in Appendix B.5 which are deferred to a different
   specification such as [I-D.ietf-6lo-ap-nd], and those related to

B.1.  Requirements Related to Mobility

   Due to the unstable nature of LLN links, even in a LLN of immobile
   nodes a 6LN may change its point of attachment to a 6LR, say 6LR-a,
   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
   and restore reachability in a timely fashion.

   Req1.1: Upon a change of point of attachment, connectivity via a new
   6LR MUST be restored timely without the need to de-register from the
   previous 6LR.

   Req1.2: For that purpose, the protocol MUST enable to differentiate
   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 capable to register its Address
   to multiple 6LRs, and this, concurrently.

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 a LLN can be based on RPL, which is the routing protocol
   that was defined at the IETF for this particular purpose.  Other
   routing protocols than RPL are also considered by Standard Defining
   Organizations (SDO) on the basis of the expected network
   characteristics.  It is required that a 6LoWPAN Node attached via ND
   to a 6LR would need to participate in the selected routing protocol
   to obtain reachability via the 6LR.

   Next to 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

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

   Related requirements are:

   Req2.1: The ND registration method SHOULD be extended in such a
   fashion that the 6LR MAY advertise the Address of a 6LoWPAN Node 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 [RFC6550] section 6.4, 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 6BBR 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 MAC-64 address.  At this point, the 6lo Working Group
   is extending the 6LoWPAN Header Compression (HC) [RFC6282] technique
   to other link types 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 IEEE1901.2 Narrowband
   Powerline Communication Networks
   [I-D.popa-6lo-6loplc-ipv6-over-ieee19012-networks] and BLUETOOTH(R)
   Low Energy [RFC7668].

   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.

   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 Neighbour Discovery should specify the formation of a
   site-local address that follows the security recommendations from

B.4.  Requirements Related to Proxy Operations

   Duty-cycled devices may not be able to answer themselves to a lookup
   from a node that uses classical ND on a Backbone and may need a
   proxy.  Additionally, the duty-cycled device may need to rely on the
   6LBR to perform registration to the 6BBR.

   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 enable a 6BBR to
   operate as a proxy to defend the Registered Addresses on its behalf.

   Req4.3: The registration mechanism SHOULD enable long sleep
   durations, in 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, the
   spoofing of the 6LR, 6LBR and 6BBRs roles 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 a LLN it makes sense to base security on layer-2 security.  During
   bootstrap of the LLN, nodes join the network after authorization by a
   Joining Assistant (JA) or a Commissioning Tool (CT).  After joining
   nodes communicate with each other via secured links.  The keys for
   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 6BBR 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 impossible.

   Req5.3: 6LoWPAN ND security mechanisms SHOULD lead to small 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

   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.

   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

   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 6LoWPAN Node that registered it
   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 and more hops.

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems, Inc
   Sophia Antipolis

   Email: pthubert@cisco.com

   Erik Nordmark
   Santa Clara, CA

   Email: nordmark@sonic.net

   Samita Chakrabarti
   San Jose, CA

   Email: samitac.ietf@gmail.com