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RFC 8505

Registration Extensions for IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor Discovery

Pages: 47
Proposed STD
Updates:  6775
Part 2 of 3 – Pages 14 to 32
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Top   ToC   Page 14   prevText
5.  Updating RFC 6775

   The EARO (see Section 4.1) updates the ARO used within NS and NA
   messages between a 6LN and a 6LR.  The update enables a registration
   to a Routing Registrar in order to obtain additional services, such
   as return routability to the Registered Address by such means as
   routing and/or proxy ND, as illustrated in Figure 4.

                 6LN            Registrar
                  |                |
                  |   NS(EARO)     |
                  |                |
                  |                | Inject/maintain
                  |                | host route or
                  |                | IPv6 ND proxy state
                  |                | <----------------->
                  |   NA(EARO)     |
                  |                |

                     Figure 4: (Re-)Registration Flow

   Similarly, the EDAR and EDAC update the DAR and DAC messages so as to
   transport the new information between 6LRs and 6LBRs across an LLN
   mesh.  The extensions to the ARO are the DAR and the DAC, as used in
   the Duplicate Address messages.  They convey the additional
   information all the way to the 6LBR.
Top   ToC   Page 15
   In turn, the 6LBR may proxy the registration to obtain reachability
   services from a Routing Registrar such as a 6BBR, as illustrated in
   Figure 5.  This specification avoids the Duplicate Address message
   flow for Link-Local Addresses in a route-over [RFC6606] topology (see
   Section 5.6).

      6LN          6LR            6LBR      Registrar
       |            |              |            |
       |<Link-local>|   <Routed>   |<Link-local>|
       |            |              |            |
       |  NS(EARO)  |              |            |
       |----------->|              |            |
       |            | Extended DAR |            |
       |            |------------->|            |
       |            |              |  proxy     |
       |            |              |  NS(EARO)  |
       |            |              |----------->|
       |            |              |            | Inject/maintain
       |            |              |            | host route or
       |            |              |            | IPv6 ND proxy state
       |            |              |            | <----------------->
       |            |              |  proxy     |
       |            |              |  NA(EARO)  |
       |            | Extended DAC |<-----------|
       |            |<-------------|            |
       |  NA(EARO)  |              |            |
       |<-----------|              |            |
       |            |              |            |

                     Figure 5: (Re-)Registration Flow

   This specification allows multiple registrations, including
   registrations for privacy and temporary addresses, and provides a
   mechanism to help clean up stale registration state as soon as
   possible, e.g., after a movement (see Section 7).

   Section 5 of [RFC6775] specifies how a 6LN bootstraps an interface
   and locates available 6LRs.  A Registering Node SHOULD register to a
   6LR that supports this specification if one is found, as discussed in
   Section 6.1, instead of registering to an RFC 6775-only 6LR;
   otherwise, the Registering Node operates in a backward-compatible
   fashion when attaching to an RFC 6775-only 6LR.
Top   ToC   Page 16
5.1.  Extending the Address Registration Option

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

   The NS message and the ARO are modified as follows:

   o  The Target Address field in the NS containing the EARO is now the
      field that indicates the address that is being registered, as
      opposed to the Source Address field in the NS as specified in
      [RFC6775] (see Section 5.5).  This change enables a 6LBR to send a
      proxy registration for a 6LN's address to a Routing Registrar and
      in most cases also avoids the use of an address as the Source
      Address before it is registered.

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

   o  The option's Length value MAY be different than 2 and take a value
      between 3 and 5, in which case the EARO is not backward compatible
      with an ARO.  The increase in size corresponds to a larger ROVR
      field, so the size of the ROVR is inferred from the option's
      Length value.

   o  A new Opaque field is introduced to carry opaque information in
      cases where the registration is relayed to another process, e.g.,
      to be advertised by a routing protocol.  A new "I" field provides
      a type for the opaque information and indicates the other process
      to which the 6LN passes the opaque value.  A value of 0 for the
      "I" field indicates topological information to be passed to a
      routing process if the registration is redistributed.  In that
      case, a value of 0 for the Opaque field (1) is backward compatible
      with the reserved fields that are overloaded and (2) indicates
      that the default topology is to be used.

   o  This document specifies a new flag in the EARO: the R flag.  If
      the R flag is set, the Registering Node requests that the 6LR
      ensure reachability for the Registered Address, e.g., by means of
      routing or proxy ND.  Conversely, when it is not set, the R flag
      indicates that the Registering Node is a router and that it will
      advertise reachability to the Registered Address via a routing
      protocol (such as RPL [RFC6550]).
Top   ToC   Page 17
   o  A node that supports this specification MUST provide a TID field
      in the EARO and set the T flag to indicate the presence of the TID
      (see Section 5.2).

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

   When registering, a 6LN that acts only as a host MUST set the R flag
   to indicate that it is not a router and that it will not handle its
   own reachability.  A 6LR that manages its reachability SHOULD NOT set
   the R flag; if it does, routes towards this router may be installed
   on its behalf and may interfere with those it advertises.

5.2.  Transaction ID

   The TID is a sequence number that is incremented by the 6LN with each
   re-registration to a 6LR.  The TID is used to determine the recency
   of the registration request.  The network uses the most recent TID to
   determine the most recent known location(s) of a moving 6LN.  When a
   Registered Node is registered with multiple 6LRs in parallel, the
   same TID MUST be used.  This enables the 6LBRs and/or Routing
   Registrars to determine whether the registrations are identical and
   to distinguish that situation from a movement (for example, see
   Section 5.7 and Appendix A).

5.2.1.  Comparing TID Values

   The operation of the TID is fully compatible with that of the RPL
   Path Sequence counter as described in Section 7.2 of [RFC6550]
   ("RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks").

   A TID is deemed to be more recent than another when its value is
   greater as determined by the operations detailed in this section.

   The TID range is subdivided in a "lollipop" fashion [Perlman83],
   where the values from 128 and greater are used as a linear sequence
   to indicate a restart and bootstrap the counter, and the values less
   than or equal to 127 are used as a circular sequence number space of
   size 128 as mentioned in [RFC1982].  Consideration is given to the
   mode of operation when transitioning from the linear region to the
   circular region.  Finally, when operating in the circular region, if
   sequence numbers are determined to be too far apart, then they are
   not comparable, as detailed below.

   A window of comparison, SEQUENCE_WINDOW = 16, is configured based on
   a value of 2^N, where N is defined to be 4 in this specification.
Top   ToC   Page 18
   For a given sequence counter,

   1.  Prior to use, the sequence counter SHOULD be initialized to an
       implementation-defined value of 128 or greater.  A recommended
       value is 240 (256 - SEQUENCE_WINDOW).

   2.  When a sequence counter increment would cause the sequence
       counter to increment beyond its maximum value, the sequence
       counter MUST wrap back to 0.  When incrementing a sequence
       counter greater than or equal to 128, the maximum value is 255.
       When incrementing a sequence counter less than 128, the maximum
       value is 127.

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

       1.  When a first sequence counter A is in the interval [128-255]
           and a second sequence counter B is in the interval [0-127]:

           1.  If (256 + B - A) is less than or equal to
               SEQUENCE_WINDOW, then B is greater than A, A is less than
               B, and the two are not equal.

           2.  If (256 + B - A) is greater than SEQUENCE_WINDOW, then A
               is greater than B, B is less than A, and the two are not

           For example, if A is 240 and B is 5, then (256 + 5 - 240) is
           21.  21 is greater than SEQUENCE_WINDOW (16); thus, 240 is
           greater than 5.  As another example, if A is 250 and B is 5,
           then (256 + 5 - 250) is 11.  11 is less than SEQUENCE_WINDOW
           (16); thus, 250 is less than 5.

       2.  In the case where both sequence counters to be compared are
           less than or equal to 127, and in the case where both
           sequence counters to be compared are greater than or equal
           to 128:

           1.  If the absolute magnitude of difference between the two
               sequence counters is less than or equal to
               SEQUENCE_WINDOW, then a comparison as described in
               [RFC1982] is used to determine the relationships
               "greater than", "less than", and "equal".

           2.  If the absolute magnitude of difference of the two
               sequence counters is greater than SEQUENCE_WINDOW, then a
               desynchronization has occurred and the two sequence
               numbers are not comparable.
Top   ToC   Page 19
   4.  If two sequence numbers are determined to be not comparable,
       i.e., the results of the comparison are not defined, then a node
       should give precedence to the sequence number that was most
       recently incremented.  Failing this, the node should select the
       sequence number in order to minimize the resulting changes to its
       own state.

5.3.  Registration Ownership Verifier (ROVR)

   The ROVR field replaces the EUI-64 field of the ARO defined in
   [RFC6775].  It is associated in the 6LR and the 6LBR with the
   registration state.  The ROVR can be a unique ID of the Registering
   Node, such as the EUI-64 address of an interface.  This can also be a
   token obtained with cryptographic methods that can be used in
   additional protocol exchanges to associate a cryptographic identity
   (key) with this registration to ensure that only the owner can modify
   it later, if the proof of ownership of the ROVR can be obtained.  The
   scope of a ROVR is the registration of a particular IPv6 Address, and
   it MUST NOT be used to correlate registrations of different

   The ROVR can be of different types; the type is signaled in the
   message that carries the new type.  For instance, the type can be a
   cryptographic string and can be used to prove the ownership of the
   registration as specified in [AP-ND] ("Address Protected Neighbor
   Discovery for Low-power and Lossy Networks").  In order to support
   the flows related to the proof of ownership, this specification
   introduces new status codes "Validation Requested" and "Validation
   Failed" in the EARO.

   Note regarding ROVR collisions: Different techniques for forming the
   ROVR will operate in different namespaces.  [RFC6775] specifies the
   use of EUI-64 addresses.  [AP-ND] specifies the generation of
   cryptographic tokens.  While collisions are not expected in the
   EUI-64 namespace only, they may happen if [AP-ND] is implemented by
   at least one of the nodes.  An implementation that understands the
   namespace MUST consider that ROVRs from different namespaces are
   different even if they have the same value.  An RFC 6775-only 6LBR or
   6LR will confuse the namespaces; this slightly increases the risk of
   a ROVR collision.  A ROVR collision has no effect if the two
   Registering Nodes register different addresses, since the ROVR is
   only significant within the context of one registration.  A ROVR is
   not expected to be unique to one registration, as this specification
   allows a node to use the same ROVR to register multiple IPv6
   Addresses.  This is why the ROVR MUST NOT be used as a key to
   identify the Registering Node or as an index to the registration.  It
   is only used as a match to ensure that the node that updates a
   registration for an IPv6 Address is the node that made the original
Top   ToC   Page 20
   registration for that IPv6 Address.  Also, when the ROVR is not an
   EUI-64 address, then it MUST NOT be used as the Interface Identifier
   of the Registered Address.  This way, a registration that uses that
   ROVR will not collide with that of an IPv6 Address derived from
   EUI-64 and using the EUI-64 as the ROVR per [RFC6775].

   The Registering Node SHOULD store the ROVR, or enough information to
   regenerate it, in persistent memory.  If this is not done and an
   event such as a reboot causes a loss of state, re-registering the
   same address could be impossible until (1) the 6LRs and the 6LBR
   time out the previous registration or (2) a management action clears
   the relevant state in the network.

5.4.  Extended Duplicate Address Messages

   In order to map the new EARO content in the EDA messages, a new TID
   field is added to the EDAR and EDAC messages as a replacement for the
   Reserved field, and a non-null value of the ICMP Code indicates
   support for this specification.  The format of the EDAR and EDAC
   messages is presented in Section 4.2.

   As with the EARO, the EDA messages are backward compatible with the
   RFC 6775-only versions, as long as the ROVR field is 64 bits long.
   Remarks concerning backward compatibility for the protocol between
   the 6LN and the 6LR apply similarly between a 6LR and a 6LBR.

5.5.  Registering the Target Address

   An NS message with an EARO is a registration if and only if it also
   carries an SLLA Option ("SLLAO") [RFC6775] ("SLLA" stands for "Source
   Link-Layer Address").  The EARO can also be used in NS and NA
   messages between Routing Registrars to determine the distributed
   registration state; in that case, it does not carry the SLLA Option
   and is not confused with a registration.

   The Registering Node is the node that performs the registration to
   the Routing Registrar.  As also described in [RFC6775], it may be the
   Registered Node as well, in which case it registers one of its own
   addresses and indicates its own MAC address as the SLLA in the

   This specification adds the capability to proxy the registration
   operation on behalf of a Registered Node that is reachable over an
   LLN mesh.  In that case, if the Registered Node is reachable from the
   Routing Registrar via a mesh-under configuration, the Registering
   Node indicates the MAC address of the Registered Node as the SLLA in
   the NS(EARO).  If the Registered Node is reachable over a route-over
   configuration from the Registering Node, the SLLA in the NS(ARO) is
Top   ToC   Page 21
   that of the Registering Node.  This enables the Registering Node to
   attract the packets from the Routing Registrar and route them over
   the LLN to the Registered Node.

   In order to enable the latter operation, this specification changes
   the behavior of the 6LN and the 6LR so that the Registered Address is
   found in the Target Address field of the NS and NA messages as
   opposed to the Source Address field.  With this convention, a TLLA
   Option (Target Link-Layer Address Option, or "TLLAO") indicates the
   link-layer address of the 6LN that owns the address.

   A Registering Node (e.g., a 6LBR also acting as a RPL root) that
   advertises reachability for the 6LN MUST place its own link-layer
   address in the SLLA Option of the registration NS(EARO) message.
   This maintains compatibility with RFC 6775-only 6LoWPAN ND.

5.6.  Link-Local Addresses and Registration

   LLN nodes are often not wired and may move.  There is no guarantee
   that a Link-Local Address will remain unique among a huge and
   potentially variable set of neighboring nodes.

   Compared to [RFC6775], this specification only requires that a
   Link-Local Address be unique from the perspective of the two nodes
   that use it to communicate (e.g., the 6LN and the 6LR in an NS/NA
   exchange).  This simplifies the DAD process in a route-over topology
   for Link-Local Addresses by avoiding an exchange of EDA messages
   between the 6LR and a 6LBR for those addresses.

   An exchange between two nodes using Link-Local Addresses implies that
   they are reachable over one hop.  A node MUST register a Link-Local
   Address to a 6LR in order to obtain further reachability by way of
   that 6LR and, in particular, to use the Link-Local Address as the
   Source Address to register other addresses, e.g., global addresses.

   If there is no collision with a previously registered address, then
   the Link-Local Address is unique from the standpoint of this 6LR and
   the registration is not a duplicate.  Two different 6LRs might claim
   the same Link-Local Address but different link-layer addresses.  In
   that case, a 6LN MUST only interact with at most one of the 6LRs.

   The exchange of EDAR and EDAC messages between the 6LR and a 6LBR,
   which ensures that an address is unique across the domain covered by
   the 6LBR, does not need to take place for Link-Local Addresses.
Top   ToC   Page 22
   When sending an NS(EARO) to a 6LR, a 6LN MUST use a Link-Local
   Address as the Source Address of the registration, whatever the type
   of IPv6 Address that is being registered.  That Link-Local Address
   MUST be either an address that is already registered to the 6LR or
   the address that is being registered.

   When a 6LN starts up, it typically multicasts an RS and receives one
   or more unicast RA messages from 6LRs.  If the 6LR can process EARO
   messages, then it places a 6CIO in its RA message with the E flag set
   as required in Section 6.1.

   When a Registering Node does not have an already-registered address,
   it MUST register a Link-Local Address, using it as both the Source
   Address and the Target Address of an NS(EARO) message.  In that case,
   it is RECOMMENDED to use an address for which DAD is not required
   (see [RFC6775]), e.g., derived from a globally unique EUI-64 address;
   using the SLLA Option in the NS is consistent with existing ND
   specifications such as [RFC4429] ("Optimistic Duplicate Address
   Detection (DAD) for IPv6").  The 6LN MAY then use that address to
   register one or more other addresses.

   A 6LR that supports this specification replies with an NA(EARO),
   setting the appropriate status.  Since there is no exchange of EDAR
   or EDAC messages for Link-Local Addresses, the 6LR may answer
   immediately to the registration of a Link-Local Address, based solely
   on its existing state and the SLLA Option that is placed in the
   NS(EARO) message as required in [RFC6775].

   A node registers its IPv6 Global Unicast Addresses (GUAs) to a 6LR in
   order to establish global reachability for these addresses via that
   6LR.  When registering with an updated 6LR, a Registering Node does
   not use a GUA as the Source Address, in contrast to a node that
   complies with [RFC6775].  For non-Link-Local Addresses, the exchange
   of EDAR and EDAC messages MUST conform to [RFC6775], but the extended
   formats described in this specification for the DAR and the DAC are
   used to relay the extended information in the case of an EARO.

5.7.  Maintaining the Registration States

   This section discusses protocol actions that involve the Registering
   Node, the 6LR, and the 6LBR.  It must be noted that the portion that
   deals with a 6LBR only applies to those addresses that are registered
   to it; as discussed in Section 5.6, this is not the case for
   Link-Local Addresses.  The registration state includes all data that
   is stored in the router relative to that registration, in particular,
   but not limited to, an NCE.  6LBRs and Routing Registrars may store
   additional registration information and use synchronization protocols
   that are out of scope for this document.
Top   ToC   Page 23
   A 6LR cannot accept a new registration when its registration storage
   space is exhausted.  In that situation, the EARO is returned in an NA
   message with a status code of "Neighbor Cache Full" (Status 2; see
   [RFC6775] and Table 1), and the Registering Node may attempt to
   register to another 6LR.

   If the registry in the 6LBR is full, then the 6LBR cannot decide
   whether a registration for a new address is a duplicate.  In that
   case, the 6LBR replies to an EDAR message with an EDAC message that
   carries a new status code indicating "6LBR Registry Saturated"
   (Table 1).  Note: This code is used by 6LBRs instead of "Neighbor
   Cache Full" when responding to a Duplicate Address message exchange
   and is passed on to the Registering Node by the 6LR.  There is no
   point in the node retrying this registration via another 6LR, since
   the problem is network-wide.  The node may abandon that address,
   de-register other addresses first to make room, or keep the address
   "tentative" [RFC4861] and retry later.

   A node renews an existing registration by sending a new NS(EARO)
   message for the Registered Address, and the 6LR MUST report the new
   registration to the 6LBR.

   A node that ceases to use an address SHOULD attempt to de-register
   that address from all the 6LRs to which it has registered the
   address.  This is achieved using an NS(EARO) message with a
   Registration Lifetime of 0.  If this is not done, the associated
   state will remain in the network until the current Registration
   Lifetime expires; this may lead to a situation where the 6LR
   resources become saturated, even if they were correctly planned to
   start with.  The 6LR may then take defensive measures that may
   prevent this node or some other nodes from owning as many addresses
   as they request (see Section 7).

   A node that moves away from a particular 6LR SHOULD attempt to
   de-register all of its addresses registered to that 6LR and register
   to a new 6LR with an incremented TID.  When/if the node appears
   elsewhere, an asynchronous NA(EARO) or EDAC message with a status
   code of "Moved" SHOULD be used to clean up the state in the previous
   location.  The "Moved" status can be used by a Routing Registrar in
   an NA(EARO) message to indicate that the ownership of the proxy state
   was transferred to another Routing Registrar due to movement of the
   device.  If the receiver of the message has registration state
   corresponding to the related address, it SHOULD propagate the status
   down the forwarding path to the Registered Node (e.g., reversing an
   existing RPL [RFC6550] path as prescribed in [Efficient-NPDAO]).
   Whether it could do so or not, the receiver MUST clean up said state.
Top   ToC   Page 24
   Upon receiving an NS(EARO) message with a Registration Lifetime of 0
   and determining that this EARO is the most recent for a given NCE
   (see Section 5.2), a 6LR cleans up its NCE.  If the address was
   registered to the 6LBR, then the 6LR MUST report to the 6LBR, through
   a Duplicate Address exchange with the 6LBR, indicating the null
   Registration Lifetime and the latest TID that this 6LR is aware of.

   Upon receiving the EDAR message, the 6LBR determines if this is the
   most recent TID it has received for that particular registry entry.
   If so, then the EDAR is answered with an EDAC message bearing a
   status code of 0 ("Success") [RFC6775], and the entry is scheduled to
   be removed.  Otherwise, a status code of "Moved" is returned instead,
   and the existing entry is maintained.

   When an address is scheduled to be removed, the 6LBR SHOULD keep its
   NCE in a DELAY state [RFC4861] for a configurable period of time, so
   as to prevent a scenario where (1) a mobile node that de-registered
   from one 6LR did not yet register to a new one or (2) the new
   registration did not yet reach the 6LBR due to propagation delays in
   the network.  Once the DELAY time has passed, the 6LBR silently
   removes its entry.

6.  Backward Compatibility

   This specification changes the behavior of the peers in a
   registration flow.  To enable backward compatibility, a 6LN that
   registers to a 6LR that is not known to support this specification
   MUST behave in a manner that is backward compatible with [RFC6775].
   Conversely, if the 6LR is found to support this specification, then
   the 6LN MUST conform to this specification when communicating with
   that 6LR.

   A 6LN that supports this specification MUST always use an EARO as a
   replacement for an ARO in its registration to a router.  This
   behavior is backward compatible, since the T flag and TID field
   occupy fields that are reserved in [RFC6775] and are thus ignored by
   an RFC 6775-only router.  A router that supports this specification
   MUST answer an NS(ARO) and an NS(EARO) with an NA(EARO).  A router
   that does not support this specification will consider the ROVR as an
   EUI-64 address and treat it the same; this scenario has no
   consequence if the Registered Addresses are different.
Top   ToC   Page 25
6.1.  Signaling EARO Support

   [RFC7400] specifies the 6CIO, which indicates a node's capabilities
   to the node's peers.  The 6CIO MUST be present in both RS and RA
   messages, unless the 6CIO information was already shared in recent
   exchanges or pre-configured in all nodes in a network.  In any case,
   a 6CIO MUST be placed in an RA message that is sent in response to an
   RS with a 6CIO.

   Section 4.3 defines a new flag for the 6CIO to signal EARO support by
   the issuer of the message.  New flags are also added to the 6CIO to
   signal the sender's capability to act as a 6LR, 6LBR, and Routing
   Registrar (see Section 4.3).

   Section 4.3 also defines a new flag that indicates the support of
   EDAR and EDAC messages by the 6LBR.  This flag is valid in RA
   messages but not in RS messages.  More information on the 6LBR is
   found in a separate Authoritative Border Router Option (ABRO).  The
   ABRO is placed in RA messages as prescribed by [RFC6775]; in
   particular, it MUST be placed in an RA message that is sent in
   response to an RS with a 6CIO indicating the capability to act as a
   6LR, since the RA propagates information between routers.

6.2.  RFC 6775-Only 6LN

   An RFC 6775-only 6LN will use the Registered Address as the Source
   Address of the NS message and will not use an EARO.  An updated 6LR
   MUST accept that registration if it is valid per [RFC6775], and it
   MUST manage the binding cache accordingly.  The updated 6LR MUST then
   use the RFC 6775-only DAR and DAC messages as specified in [RFC6775]
   to indicate to the 6LBR that the TID is not present in the messages.

   The main difference from [RFC6775] is that the exchange of DAR and
   DAC messages for the purpose of DAD is avoided for Link-Local
   Addresses.  In any case, the 6LR MUST use an EARO in the reply and
   can use any of the status codes defined in this specification.

6.3.  RFC 6775-Only 6LR

   An updated 6LN discovers the capabilities of the 6LR in the 6CIO in
   RA messages from that 6LR; if the 6CIO was not present in the RA,
   then the 6LR is assumed to be RFC 6775-only.

   An updated 6LN MUST use an EARO in the request, regardless of the
   type of 6LR -- RFC 6775-only or updated; this implies that the T flag
   is set.  It MUST use a ROVR of 64 bits if the 6LR is RFC 6775-only.
Top   ToC   Page 26
   If an updated 6LN moves from an updated 6LR to an RFC 6775-only 6LR,
   the RFC 6775-only 6LR will send an RFC 6775-only DAR message, which
   cannot be compared with an updated one for recency.  Allowing
   RFC 6775-only DAR messages to update a state established by the
   updated protocol in the 6LBR would be an attack vector; therefore,
   this cannot be the default behavior.  But if RFC 6775-only and
   updated 6LRs coexist temporarily in a network, then it makes sense
   for an administrator to install a policy that allows this behavior,
   using some method that is out of scope for this document.

6.4.  RFC 6775-Only 6LBR

   With this specification, the Duplicate Address messages are extended
   to transport the EARO information.  As with the NS/NA exchange, an
   updated 6LBR MUST always use the EDAR and EDAC messages.

   Note that an RFC 6775-only 6LBR will accept and process an EDAR
   message as if it were an RFC 6775-only DAR, as long as the ROVR is
   64 bits long.  An updated 6LR discovers the capabilities of the 6LBR
   in the 6CIO in RA messages from the 6LR; if the 6CIO was not present
   in any RA, then the 6LBR is assumed to be RFC 6775-only.

   If the 6LBR is RFC 6775-only, the 6LR MUST use only the 64 leftmost
   bits of the ROVR and place the result in the EDAR message to maintain
   compatibility.  This way, the support of DAD is preserved.

7.  Security Considerations

   This specification extends [RFC6775], and the Security Considerations
   section of that document also applies to this document.  In
   particular, the link layer SHOULD be sufficiently protected to
   prevent rogue access.

   [RFC6775] does not protect the content of its messages and expects
   lower-layer encryption to defeat potential attacks.  This
   specification requires the LLN MAC layer to provide secure unicast
   to/from a Routing Registrar and secure broadcast or multicast from
   the Routing Registrar in a way that prevents tampering with or
   replaying the ND messages.

   This specification recommends using privacy techniques (see
   Section 8) and protecting against address theft via methods that are
   outside the scope of this document.  As an example, [AP-ND]
   guarantees the ownership of the Registered Address using a
   cryptographic ROVR.
Top   ToC   Page 27
   The registration mechanism may be used by a rogue node to attack the
   6LR or 6LBR with a denial-of-service attack against the registry.  It
   may also happen that the registry of a 6LR or 6LBR is saturated and
   cannot take any more registrations; this scenario effectively denies
   the requesting node the capability to use a new address.  In order to
   alleviate those concerns, (1) Section 5.2 provides a sequence counter
   that keeps incrementing to detect and clean up stale registration
   information and that contributes to defeat replay attacks and
   (2) Section 5.7 provides a number of recommendations that ensure that
   a stale registration is removed as soon as possible from the 6LR
   and 6LBR.

   In particular, this specification recommends that:

   o  A node that ceases to use an address SHOULD attempt to de-register
      that address from all the 6LRs to which it is registered.

   o  The registration lifetimes SHOULD be individually configurable for
      each address or group of addresses.  A node SHOULD be configured
      for each address (or address category) with a Registration
      Lifetime that reflects the expectation of how long it will use the
      address with the 6LR to which the address is registered.  In
      particular, use cases that involve mobility or rapid address
      changes SHOULD use lifetimes that are the same order of magnitude
      as the duration of the expectation of presence but that are still

   o  The router (6LR or 6LBR) SHOULD be configurable so as to limit the
      number of addresses that can be registered by a single node, but
      as a protective measure only.  In any case, a router MUST be able
      to keep a minimum number of addresses per node.  That minimum
      depends on the type of device and ranges between 3 for a very
      constrained LLN and 10 for a larger device.  A node may be
      identified by its MAC address, as long as it is not obfuscated by
      privacy measures.  A stronger identification (e.g., by security
      credentials) is RECOMMENDED.  When the maximum is reached, the
      router SHOULD use a Least Recently Used (LRU) algorithm to
      clean up the addresses, keeping at least one Link-Local Address.
      The router SHOULD attempt to keep one or more stable addresses if
      stability can be determined, e.g., because they are used over a
      much longer time span than other (privacy, shorter-lived)

   o  In order to avoid denial of registration due to a lack of
      resources, administrators should take great care to deploy
      adequate numbers of 6LRs to cover the needs of the nodes in their
      range, so as to avoid a situation of starving nodes.  It is
      expected that the 6LBR that serves an LLN is a more capable node
Top   ToC   Page 28
      than the average 6LR, but in a network condition where it may
      become saturated, a particular LLN should distribute the 6LBR
      functionality -- for instance, by leveraging a high-speed Backbone
      Link and Routing Registrars to aggregate multiple LLNs into a
      larger subnet.

   The LLN nodes depend on a 6LBR and may use the services of a Routing
   Registrar for their operation.  A trust model MUST be put in place to
   ensure that only authorized devices are acting in these roles, so as
   to avoid threats such as black-holing or bombing attack whereby an
   impersonated 6LBR would destroy state in the network by using the
   "Removed" status code.  At a minimum, this trust model could be based
   on Layer 2 access control or could provide role validation as well
   (see Req-5.1 in Appendix B.5).

8.  Privacy Considerations

   As indicated in Section 3, this protocol does not limit the number of
   IPv6 Addresses that each device can form.  However, to mitigate
   denial-of-service attacks, it can be useful as a protective measure
   to have a limit that is high enough not to interfere with the normal
   behavior of devices in the network.  A host should be able to form
   and register any address that is topologically correct in the
   subnet(s) advertised by the 6LR/6LBR.

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

   [RFC8065] ("Privacy Considerations for IPv6 Adaptation-Layer
   Mechanisms") explains why privacy is important and how to form
   privacy-aware addresses.  All implementations and deployments must
   consider the option of privacy addresses in their own environments.

   The IPv6 Address of the 6LN in the IPv6 header can be compressed
   statelessly when the Interface Identifier in the IPv6 Address can be
   derived from the lower-layer address.  When it is not critical to
   benefit from that compression, e.g., the address can be compressed
   statefully, or it is rarely used and/or it is used only over one hop,
   privacy concerns should be considered.  In particular, new
   implementations should follow [RFC8064] ("Recommendation on Stable
   IPv6 Interface Identifiers").  [RFC8064] recommends the mechanism
   specified in [RFC7217] ("A Method for Generating Semantically Opaque
   Interface Identifiers with IPv6 Stateless Address Autoconfiguration
   (SLAAC)") for generating Interface Identifiers to be used in SLAAC.
Top   ToC   Page 29
9.  IANA Considerations

   IANA has made a number of changes under the "Internet Control Message
   Protocol version 6 (ICMPv6) Parameters" registry, as follows.

9.1.  Address Registration Option Flags

   IANA has created a new subregistry for "Address Registration Option
   Flags" under the "Internet Control Message Protocol version 6
   (ICMPv6) Parameters" registry.  (See [RFC4443] for information
   regarding ICMPv6.)

   This specification defines eight positions -- bit 0 to bit 7 -- and
   assigns bit 6 for the R flag and bit 7 for the T flag (see
   Section 4.1).  The registration procedure is "IETF Review" or "IESG
   Approval" (see [RFC8126]).

   The initial contents of the registry are shown in Table 2.

                |  ARO Status | Description  | Reference  |
                |     0-5     | Unassigned   |            |
                |             |              |            |
                |      6      | R Flag       | RFC 8505   |
                |             |              |            |
                |      7      | T Flag       | RFC 8505   |

              Table 2: New Address Registration Option Flags

9.2.  Address Registration Option I-Field

   IANA has created a new subregistry for "Address Registration Option
   I-Field" under the "Internet Control Message Protocol version 6
   (ICMPv6) Parameters" registry.

   This specification defines four integer values from 0 to 3 and
   assigns value 0 to "Abstract Index for Topology Selection" (see
   Section 4.1).  The registration procedure is "IETF Review" or "IESG
   Approval" [RFC8126].
Top   ToC   Page 30
   The initial contents of the registry are shown in Table 3.

      | Value  | Meaning                               | Reference  |
      | 0      | Abstract Index for Topology Selection | RFC 8505   |
      |        |                                       |            |
      | 1-3    | Unassigned                            |            |

               Table 3: New Subregistry for the EARO I-Field

9.3.  ICMP Codes

   IANA has created two new subregistries of the 'ICMPv6 "Code" Fields'
   registry, which itself is a subregistry of ICMPv6 codes in the
   "Internet Control Message Protocol version 6 (ICMPv6) Parameters"

   The new subregistries relate to ICMP Types 157 (Duplicate Address
   Request) (shown in Table 4) and 158 (Duplicate Address Confirmation)
   (shown in Table 5), respectively.  For those two ICMP types, the ICMP
   Code field is split into two subfields: the Code Prefix and the Code
   Suffix.  The new subregistries relate to the Code Suffix portion of
   the ICMP Code.  The range of the Code Suffix is 0-15 in all cases.
   The registration procedure is "IETF Review" or "IESG Approval"
   [RFC8126] for both subregistries.

   The initial contents of these subregistries are as follows:

   | Code Suffix  | Meaning                              | Reference  |
   | 0            | DAR message                          | RFC 6775   |
   |              |                                      |            |
   | 1            | EDAR message with 64-bit ROVR field  | RFC 8505   |
   |              |                                      |            |
   | 2            | EDAR message with 128-bit ROVR field | RFC 8505   |
   |              |                                      |            |
   | 3            | EDAR message with 192-bit ROVR field | RFC 8505   |
   |              |                                      |            |
   | 4            | EDAR message with 256-bit ROVR field | RFC 8505   |
   |              |                                      |            |
   | 5-15         | Unassigned                           |            |

           Table 4: Code Suffixes for ICMP Type 157 DAR Message
Top   ToC   Page 31
   | Code Suffix  | Meaning                              | Reference  |
   | 0            | DAC message                          | RFC 6775   |
   |              |                                      |            |
   | 1            | EDAC message with 64-bit ROVR field  | RFC 8505   |
   |              |                                      |            |
   | 2            | EDAC message with 128-bit ROVR field | RFC 8505   |
   |              |                                      |            |
   | 3            | EDAC message with 192-bit ROVR field | RFC 8505   |
   |              |                                      |            |
   | 4            | EDAC message with 256-bit ROVR field | RFC 8505   |
   |              |                                      |            |
   | 5-15         | Unassigned                           |            |

           Table 5: Code Suffixes for ICMP Type 158 DAC Message

9.4.  New ARO Status Values

   IANA has made additions to the "Address Registration Option Status
   Values" subregistry, as follows:

    | Value | Description                                | Reference  |
    |   3   | Moved                                      | RFC 8505   |
    |       |                                            |            |
    |   4   | Removed                                    | RFC 8505   |
    |       |                                            |            |
    |   5   | Validation Requested                       | RFC 8505   |
    |       |                                            |            |
    |   6   | Duplicate Source Address                   | RFC 8505   |
    |       |                                            |            |
    |   7   | Invalid Source Address                     | RFC 8505   |
    |       |                                            |            |
    |   8   | Registered Address Topologically Incorrect | RFC 8505   |
    |       |                                            |            |
    |   9   | 6LBR Registry Saturated                    | RFC 8505   |
    |       |                                            |            |
    |   10  | Validation Failed                          | RFC 8505   |

                      Table 6: New ARO Status Values
Top   ToC   Page 32
9.5.  New 6LoWPAN Capability Bits

   IANA has made additions to the "6LoWPAN Capability Bits" subregistry,
   as follows:

             | Bit  | Description               | Reference  |
             |  10  | EDA Support (D bit)       | RFC 8505   |
             |      |                           |            |
             |  11  | 6LR capable (L bit)       | RFC 8505   |
             |      |                           |            |
             |  12  | 6LBR capable (B bit)      | RFC 8505   |
             |      |                           |            |
             |  13  | Routing Registrar (P bit) | RFC 8505   |
             |      |                           |            |
             |  14  | EARO support (E bit)      | RFC 8505   |

                   Table 7: New 6LoWPAN Capability Bits

(page 32 continued on part 3)

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