6lo                                                      P. Thubert, Ed.
Internet-Draft                                                     cisco
Updates: 6775 (if approved)                                  E. Nordmark
Intended status: Standards Track
Expires: November 13, December 23, 2017                                S. Chakrabarti
                                                            May 12,
                                                           June 21, 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, and as well as to
   provide enhancements to the registration capabilities, in particular capabilities and mobility
   detection for different network topologies including the
   registration to a Backbone Router for backbone
   routers performing proxy ND operations. 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
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   This Internet-Draft will expire on November 13, December 23, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Considerations On  Applicability of Address Registration Rejection  . . Options . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Updating RFC 6775 . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Extended Address Registration Option  . . . . . . . . . .   5   6
     4.2.  Transaction ID  . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Owner Unique ID . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  Registering the Target Address  . . . . . . . . . . . . .   7
     4.5.  Link-Local Addresses and Registration . . . . . . . . . .   8
     4.6.  Maintaining the Registration States . . . . . . . . . . .   9
   5.  Extending RFC 7400  . . . . . . . . . . .  Detecting Enhanced ARO Capability Support . . . . . . . . . .  11  10
   6.  Updated ND Options  . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  The Enhanced Address Registration Option (EARO) . . . . .  11
     6.2.  New 6LoWPAN capability Bits in the Capability Indication
           Option  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .  14
     7.1.  Discovering the capabilities of an ND peer  . . . . . . .  14
       7.1.1.  Using the E Flag in the CIO . . . . . . . . . . . . .  14
       7.1.2.  Using the T Flag in the EARO  . . . . . . . . . . . .  15
     7.2.  Legacy 6LoWPAN Node . . . . . . . . . . . . . . . . . . .  15
     7.3.  Legacy 6LoWPAN Router . . . . . . . . . . . . . . . . . .  16
     7.4.  Legacy 6LoWPAN Border Router  . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  18
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   11.  20
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     12.2.  Informative References . . . . . . . . . . . . . . . . .  21
     12.3.  External Informative References  . . . . . . . . . . . .  24  23
   Appendix A.  Applicability and Requirements Served  . . . . . . .  24
   Appendix B.  Requirements . . . . . . . . . . . . . . . . . . . .  25  24
     B.1.  Requirements Related to Mobility  . . . . . . . . . . . .  25
     B.2.  Requirements Related to Routing Protocols . . . . . . . .  25
     B.3.  Requirements Related to the Variety of Low-Power Link
           types . . . . . . . . . . . . . . . . . . . . . . . . . .  26
     B.4.  Requirements Related to Proxy Operations  . . . . . . . .  27
     B.5.  Requirements Related to Security  . . . . . . . . . . . .  27
     B.6.  Requirements Related to Scalability . . . . . . . . . . .  29  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   RFC 6775, the "Neighbor Discovery Optimization for IPv6 over Low-
   Power Wireless Personal Area Networks (6LoWPANs)" [RFC6775]
   introduced a proactive registration mechanism to IPv6 Neighbor
   Discovery (ND) services that is well suited to nodes belonging to a
   Low Power Lossy Network (LLN).

   The scope of this draft is an IPv6 LLN, which can be a simple Low Power Networks including star or
   a more complex
   and mesh topology.  The LLN may be anchored at an IPv6
   Backbone Router (6BBR) [I-D.ietf-6lo-backbone-router]. topologies.  This specification modifies and extends the
   behavior and protocol elements of RFC 6775 "Neighbor Discovery
   Optimization for IPv6 over Low-Power Wireless Personal Area Networks
   (6LoWPANs)" [RFC6775] to enable additional capabilities, in
   particular capabilities such as:

      * Support the indication of mobility vs retry (T-bit)

      * Ease up requirement of registration for link-local addresses

      * Introducing Enhancement to a 6BBR Address Registration Option (ARO)

      * Permitting regitration of target address

      * Clarification of support of privacy and temporary addresses

   The following sections will discuss applicability of 6LoWPAN ND
   registration, new extensions and updates to RFC 6775.  Finally, we
   will discuss how the extensions of registration framework can be
   useful for a scenario such as Backbone router(6BBR) proxy ND

2.  Considerations On  Applicability of Address Registration Rejection Options

   The purpose of the Address Registration Option (ARO) [RFC6775] and of
   the Extended ARO (EARO) that is introduced in this document is to
   facilitate duplicate address detection (DAD) for hosts and pre-
   populate Neighbor Cache Entries (NCE) [RFC4861] in the routers to
   reduce the need for sending multicast 'multicast neighbor solicitations and also
   to solicitations' which
   may be able to support IPv6 Backbone Routers. harmful in low power constrained nodes networks where
   multicast is most often treated as broadcasts.

   In some cases the address registration can fail or be becomes useless
   for reasons other than a duplicate address.  Examples are the router
   having run out of space, a registration bearing a stale sequence
   number (e.g. denoting a movement of the host after this registration
   was placed), a host misbehaving and attempting to register an invalid
   address such as the unspecified address [RFC4291], or the 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 6LR, depending on the returned error.

   However, the ability to return errors to address registrations MUST
   NOT be used to restrict the ability of hosts to form and use
   addresses as recommended in "Host Address Availability
   Recommendations" [RFC7934].  In particular, this is needed for
   enhanced privacy, which implies that each host will register a
   multiplicity of address as part mechanisms like "Privacy Extensions
   for Stateless Address Autoconfiguration (SLAAC) in IPv6" [RFC4941].
   This implies that the capabilities of 6LR and 6LBRs in terms of
   number of registrations must be clearly announced in the router
   documentation, and that 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.

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

      "Neighbor Discovery for IP version 6" [RFC4861],

      "IPv6 Stateless Address Autoconfiguration" [RFC4862],

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

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

      "Multi-link Subnet Support in IPv6"

   Additionally, this document uses terminology from

      "Terms Used in Routing for Low-Power and Lossy Networks" [RFC7102]

      the "6TiSCH Terminology" [I-D.ietf-6tisch-terminology],

   as well as this additional terminology:

   Backbone  This is an IPv6 transit link that interconnects 2 or more
         Backbone Routers.  It is expected to be deployed as a high
         speed Backbone in order to federate a potentially large set of
         LLNS.  Also referred to as a LLN Backbone or Backbone network.

   Backbone Router  An IPv6 router that federates the LLN using a
         Backbone link as a Backbone.  A 6BBR acts as a 6LoWPAN Border
         Routers (6LBR) and an Energy Aware Default Router (NEAR).

   Extended LLN  This is 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 proxy ND for
         it over the Backbone.

   Binding  The state in the 6BBR that associates an IP address with a
         MAC address, a port and some other information about the node
         that owns the IP address.

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

   Registering Node  The node that performs the registration to the
         6BBR, either for one of its own addresses, in which case it is
         Registered Node and indicates its own MAC Address as Source
         Link Layer Address (SLLA) in the NS(EARO), or on behalf of a
         Registered Node that is reachable over a LLN mesh.  In the
         latter 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).
         Otherwise, it is expected that the Registered Device is
         reachable over a Route-Over mesh from the Registering Node, in
         which case the SLLA in the NS(ARO) is that of the Registering
         Node, which causes it to attract the packets from the 6BBR to
         the Registered Node and route them over the LLN.

   Registered Address  The address owned by the Registered Node node
         that is being registered.

4.  Updating RFC 6775

   This specification extends the Address Registration Option (ARO)
   defined in RFC 6775 [RFC6775]; in particular a "T" flag is added that
   must be set is NS messages when this specification is used, and
   echo'ed in NA messages to confirm that the protocol effectively
   supported.  Support for this specification can thus be inferred from
   the presence of the Extended ARO ("T" flag set) in ND messages.

   In order to support various types of link layers, this specification
   also adds recommendation 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 will favor
   registering to a 6LR that indicates support for this specification
   over that of RFC 6775 [RFC6775].

4.1.  Extended Address Registration Option

   This specification extends the ARO option that is used for the
   process of address registration.  The new ARO is referred to as
   Extended ARO (EARO), and its semantics are modified as follows:

   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 4.4 for
   more).  This change enables a 6LBR to use an address of his 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 Duplicate Address
   Detection (DAD) is complete.

   The Unique ID in the EARO option does no more have to be a MAC
   address (see Section 4.3 for more).  This enables in particular the
   use of a Provable Temporary UID (PT-UID) as opposed to burn-in MAC
   address, the PT-UID providing a trusted anchor by the 6LR and 6LBR to
   protect the state associated to the node.

   The specification introduces a Transaction ID (TID) field in the EARO
   (see Section 4.2 for more on TID).  The TID MUST be provided by a
   node that supports this specification and a new T flag MUST be set to
   indicate so.  The T bit can be used to determine whether the peer
   supports this specification.

   Finally, this specification introduces a number of new Status codes
   to help diagnose the cause of a registration failure (more in
   Table 1).

4.2.  Transaction ID

   The specification expects that the Registered Node can provide a
   sequence number called Transaction ID (TID) that is incremented with
   each re-registration.  The TID essentially obeys is used to detect the same rules as
   the Path Sequence field in the Transit Information Option (TIO) found
   in the RPL Destination Advertisement Object (DAO) [RFC6550].  This
   way, the LLN node can use freshness of the same counter for ND and RPL,
   registration request and a 6LBR
   acting as RPL root may easily maintain the useful to detect one single registration on behalf of
   a RPL node deep inside the mesh by simply using
   multiple 6LOWPAN border routers supporting the RPL TIO Path
   Sequence same large 6LOWPAN, as TID
   is the case for EARO.

   When backbone routers (BBR).

   For example, when a Registered Node is registered to with multiple BBRs
   in parallel, it is expected that the same TID is used, to enable the
   6BBRs to correlate the registrations as being a single one, and
   differentiate that situation from a movement.

   If the TIDs are different, a conflict resolution inherited from RPL
   sorts out the most recent registration and other ones are removed.
   The operation for computing and comparing the Path Sequence is
   detailed in section 7 of RFC 6550 [RFC6550] and applies to the

   Thus TID in
   the exact same fashion.  The resolution is used could be tracked to determine follow the
   freshest registration for sequence of mobility of a particular address, and an EARO is
   processed only if it is
   node.  The details protocols of mobility verification by the freshest, otherwise a Status code 3
   "Moved" border
   routers is returned. not part of this specification.

4.3.  Owner Unique ID

   The Owner Unique ID (OUID) enables to differentiate a real duplicate
   address registration 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 and if they relate to a same target then
   they reflect an address duplication.  The Owner Unique ID can be as
   simple as a EUI-64 burn-in address, if duplicate EUI-64 addresses are

   Alternatively, the unique ID can be a cryptographic string that can
   can be 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 after a reboot that
   would cause a loss of memory until the Backbone Router times out the

4.4.  Registering the Target Address

   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 in Route-Over meshes, for instance to enable that a
   RPL root registers addresses on behalf LLN nodes that are deeper in a
   6TiSCH mesh, 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 get the packets and route them
   down the mesh.  But the Registered Address belongs to another node,
   the Registered Node, and that address is indicated in the Target
   Address field of the NS message.

   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.

   Since the Registering Node is the one that has reachability with the
   6LR, and is the one expecting packets for the 6LN, it makes sense to
   maintain compatibility with RFC 6775 [RFC6775], and it is REQUIRED
   that an SLLA Option is always placed in a registration NS(EARO)

4.5.  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 [RFC6775], this specification only requires that
   a Link-Local address is unique from the perspective of the peering
   nodes.  This simplifies the Duplicate Address Detection (DAD) for
   Link-Local addresses, and there is no DAR/DAC Duplicate Address Request (DAR)
   / Duplicate Address Confirmation (DAC) exchange between the 6LR and a
   6LBR for Link-Local addresses.

   Additionally, RFC 6775 [RFC6775] requires that a 6LoWPAN Node (6LN)
   uses 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 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 removed.

   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 a
   same Link-Local address but different link-layer addresses.  In that
   case, a 6LN may only interact with one of the 6LR so as to avoid
   confusion in the 6LN neighbor cache.

   The DAD process between the 6LR and a 6LoWPAN Border Router (6LBR),
   which is based on a Duplicate Address Request (DAR) / Duplicate
   Address Confirmation (DAC) exchange as described in RFC 6775
   [RFC6775], 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 DAR/DAC 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

   A node needs to register its IPv6 Global Unicast IPv6 Addresses (GUA)
   to a 6LR in order to obtain a global reachability for these addresses
   via that 6LR.  As opposed to a node that complies to RFC 6775
   [RFC6775], a Registering Node registering a GUA does not use that GUA
   as Source Address for the registration to a 6LR that conforms this
   specification.  The DAR/DAC exchange MUST take place for non-Link-
   Local addresses as prescribed by RFC 6775 [RFC6775].

4.6.  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 4.5, 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 6LBR cannot guarantee that a new address
   is effectively not a duplicate.  In that case, the 6LBR replies to a
   DAR message with a DAC message that carries a Status code 9
   indicating "6LBR Registry saturated", and the address stays in
   TENTATIVE state.

   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.  This is normally done through a DAR/DAC exchange, but
   the refresh MAY alternatively be piggy-backed in another protocol
   such as RPL [RFC6550], as long as the semantics of the EARO are fully
   carried in the alternate protocol.  In the particular case of RPL,
   the TID MUST be used as the Path Sequence in the TIO, and the
   Registration Lifetime MUST be used as Path Lifetime.  It is also
   REQUIRED that the root of the RPL DODAG passes that information to
   the 6LBR on behalf of the 6LR, either through a DAR/DAC exchange, or
   through internal methods if they are collocated.

   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.

   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 DAR/DAC
   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 DAR message, the 6LBR evaluates if this is the freshest EARO
   it has received for that particular registry entry.  If it is, then
   the entry is scheduled to be removed, and the DAR is answered with a
   DAC message bearing a Status of 0 "Success".  If it is not the
   freshest, then a Status 2 "Moved" is returned instead, and the
   existing entry is conserved.  The 6LBR SHOULD conserve the address 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.

5.  Extending RFC 7400

   RFC 7400 [RFC7400] introduces the 6LoWPAN  Detecting Enhanced ARO Capability Indication
   Option (6CIO) to indicate Support

   The nodes and routers in a node's capabilities to its peers.  This
   specification extends network may be mixed and if a node wants
   to use EARO feature for address registration, it has to find a router
   which supports it.  Thus all implementations with EARO option MUST
   provide the capability detection method using 6CIO option to support
   both types of registrations (ARO and EARO) as described in later
   sections.  Moreover, any new implementation of 6LOWPAN is also
   RECOMMENDED to support 6LoWPAN Capability Indication option(6CIO)in

   RFC 7400 [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 capability to act as a 6LR, 6LBR and

   With RFC 7400 [RFC7400], the 6CIO is typically sent Router
   Solicitation (RS) messages.  When used to signal the capabilities
   above per this specification, the 6CIO is typically present Router
   Advertisement (RA) messages but can also be present in RS, Neighbor
   Solicitation (NS) and Neighbor Advertisement (NA) messages.

6.  Updated ND Options

   This specification does not introduce new options, but it modifies
   existing ones and updates the associated behaviors as follow:

6.1.  The Enhanced Address Registration Option (EARO)

   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 LLN node and its 6LoWPAN Router (6LR), as
   well as in Duplicate Address Request (DAR) and the Duplicate Address
   Confirmation (DAC) messages between 6LRs and 6LBRs in LLNs meshes
   such as 6TiSCH networks.

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

   The EARO extends the ARO and is recognized by the "T" flag set.

   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 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 1: EARO

   Option Fields

   Type:  33

   Length:  8-bit unsigned integer.

   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.

   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.

   TID:  1-byte integer; a transaction id that is maintained by the node
      and incremented with each transaction.  it is recommended that the
      node maintains the TID in a persistent storage.

   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 cryptographically.

   | Value | Description                                               |
   |  0..2 | See RFC 6775 [RFC6775].  Note that 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   | Proof 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 time out of a lifetime, or a movement.    |
   |       | The receiver of the NA is the device that has performed a |
   |       | registration that is now stale and it should clean up its |
   |       | state.                                                    |
   |       |                                                           |
   |   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.  This          |
   |       |                                                           |
   |   10  | Incorrect proof: The proof of ownership of the registered |
   |       | address is not correct.                                   |

                           Table 1: EARO Status

   Note: the code "6LBR Registry saturated" is used by 6LBRs instead of
   Status 2 when responding |
   |       | to a DAR/DAC 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.                                              |

                           Table 1: EARO Status

6.2.  New 6LoWPAN capability Bits in the Capability Indication Option

   This specification defines a number of capability bits in the CIO
   that was introduced by RFC 7400 [RFC7400].

   Support for this specification is indicated by setting the "E" flag
   in a CIO option.  Routers that are capable of acting as 6LR, 6LBR and
   6BBR SHOULD set the L, B and P flags, respectively.

   Those flags are not mutually exclusive and if a router is capable of
   multiple roles, 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  |_____________________|L|B|P|E|G|

            Figure 2: New capability Bits L, B, P, E in the CIO

   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 Flag in the CIO

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

   It is RECOMMENDED that a router that supports this specification
   indicates so with a CIO option, but this might not be practical if
   the link-layer MTU is too small.

   If the Registering Node receives a CIO in a RA, then the setting of
   the E" Flag indicates whether or not this specification is supported.

   A node which does not implement this draft or parse 6CIO option, MUST
   ignore the packet and the sender of option SHOULD use legacy
   registration method according to RFC 6775 [RFC6775] after a timeout

7.1.2.  Using the 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 amenable 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 this 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, already 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
   [RFC6775] are ignored by a legacy router.  A router that supports
   this specification answers to an ARO with an ARO and to an EARO with
   an EARO.

   This specification changes the behavior of the peers in a
   registration flows.  To enable backward compatibility, a node that
   registers to a router that is not known to support this specification
   MUST behave as prescribed by RFC 6775.  Once the router is known to
   support this specification, the node 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.  In order to be backward compatible, an updated
   6LR needs to accept that registration if it is valid per the RFC 6775
   [RFC6775] specification, and manage the binding cache accordingly.

   The main difference with RFC 6775 is that DAR/DAC exchange for DAD
   may be avoided for Link-Local addresses.  Additionally, 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 a an updated 6LN is for a Link-Local
   address, using that Link-Local address as source.  A legacy 6LN will
   not makes 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 6LN will always areply 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.

   When facing a legacy 6LR, an updated 6LN may attempt to find an
   alternate 6LR that is updated.  In order to be backward compatible,
   based on the discovery that a 6LR is legacy, the 6LN needs to
   fallback to legacy behavior and source the packet with the Registered

   The main difference is that the updated 6LN SHOULD use an EARO in the
   request regardless of the type of 6LN, legacy or updated

7.4.  Legacy 6LoWPAN Border Router

   With this specification, the DAR/DAC transports an EARO option as
   opposed to an ARO option.  As described for the NS/NA exchange,
   devices that support this specification always use an EARO option and
   all the associated behavior.

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

   This specification does not mandate any particular way for forming
   IPv6 addresses, but it recognizes that use of EUI-64 for forming the
   Interface ID in the Link-Local address prevents the usage of "SEcure
   Neighbor Discovery (SEND)" [RFC3971] and "Cryptographically Generated
   Addresses (CGA)" [RFC3972], and that of address recommends to using privacy techniques,
   such as recommended techniques (more in "Privacy Considerations for IPv6 Adaptation-
   Layer Mechanisms" [RFC8065].  This specification RECOMMENDS the use
   of privacy techniques,
   section Section 9, and that of additional 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.

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

   On the other hand, the

   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 4.6 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 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 homogeneous with 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, 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.

   o  Administrators should take great care to deploy adequate numbers
      of 6LR 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.

   When the ownership of the OUID cannot be assessed, this specification
   limits the cases where the OUID and the TID are multicasted, and
   obfuscates them in responses to attempts to take over an address.

   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 recognizes that use of EUI-64 for forming the
   Interface ID in the Link-Local address 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] addresses 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 create a new subregistry for "ARO Flags" under
   the "Internet Control Message Protocol version 6 (ICMPv6)
   Parameters".  This specification defines 8 positions, bit 0 to bit 7,
   and assigns bit 7 for the "T" flag in Section 6.1.  The policy is
   "IETF Review" or "IESG Approval" [RFC5226].  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) Parameters"

                 | ARO Status | Description  | Document  |
                 |    0..6    | Unassigned   |           |
                 |            |              |           |
                 |     7      | "T" Flag     | RFC This  |

                          Table 2: new ARO Flags

   IANA is requested to make additions to existing registries as

            Address Registration Option Status Values Registry

   | ARO Status | Description                              | Document  |
   |     3      | Moved                                    | RFC This  |
   |            |                                          |           |
   |     4      | Removed                                  | RFC This  |
   |            |                                          |           |
   |     5      | Proof 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     | Incorrect proof                          | RFC This  |

                      Table 3: New ARO Status values

   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 4: New 6LoWPAN capability Bits


11.  Acknowledgments

   Kudos to Eric Levy-Abegnoli who designed the First Hop Security
   infrastructure at Cisco.


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, <http://www.rfc-editor.org/info/rfc4291>.

   [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,

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [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, <http://www.rfc-editor.org/info/rfc7400>.


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-00 draft-ietf-6lo-ap-nd-02 (work in progress),
              November 2016. May

              Thubert, P., "IPv6 Backbone Router", draft-ietf-6lo-
              backbone-router-03 (work in progress), January 2017.

              Choi, Y., Hong, Y., Youn, J., Kim, D., and J. Choi,
              "Transmission of IPv6 Packets over Near Field
              Communication", draft-ietf-6lo-nfc-06 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-11 (work
              in progress), January 2017.

              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terminology in IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-terminology-08 (work in
              progress), December 2016.

              Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
              S. Aldrin, "Multicast using Bit Index Explicit
              Replication", draft-ietf-bier-architecture-06 draft-ietf-bier-architecture-07 (work in
              progress), April June 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.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
              2003, <http://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,

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <http://www.rfc-editor.org/info/rfc7102>.

   [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, <http://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, <http://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, <http://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,

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