ROLL                                                     P. Thubert, Ed.
Internet-Draft                                                     Cisco
Updates: 6550, 6775 (if approved)                      February 23, 2018
Intended status: Standards Track
Expires: August 27, 2018

                         Routing for RPL Leaves


   This specification updates RFC 6550 and RFC 6775 unicast routing
   service in a RPL domain to 6LoWPAN ND nodes that do not participate
   to the routing protocol.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Updating RFC 6550 . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Updating RFC 6775 Update  . . . . . . . . . . . . . . . . . .   5
   5.  Protocol Operations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  General Flow  . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  6LN Operation . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  6LR Operation . . . . . . . . . . . . . . . . . . . . . .   7
     5.4.  RPL Root Operation  . . . . . . . . . . . . . . . . . . .   8
     5.5.  6LBR Operation  . . . . . . . . . . . . . . . . . . . . .   9
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Subset of a 6LoWPAN Glossary . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The design of Low Power and Lossy Networks (LLNs) is generally
   focused on saving energy, which is the most constrained resource of
   all.  Other design constraints, such as a limited memory capacity,
   duty cycling of the LLN devices and low-power lossy transmissions,
   derive from that primary concern.

   The IETF produced the "Routing Protocol for Low Power and Lossy
   Networks" [RFC6550] (RPL) to provide routing services within such
   constraints.  RPL is a Distance-Vector protocol, which, compared to
   link-state protocols, limits the amount of topological knowledge that
   needs to be installed and maintained in each node.  In order to
   operate in constrained networks, RPL allows a Routing Stretch (see
   [RFC6687]), whereby routing is only performed along a DODAG as
   opposed to straight along a shortest path between 2 peers, whatever
   that would mean in a given LLN.  This trades the quality of peer-to-
   peer (P2P) paths for a vastly reduced amount of control traffic and
   routing state that would be required to operate a any-to-any shortest
   path protocol.  Finally, broken routes may be fixed lazily and on-
   demand, based on dataplane inconsistency discovery, which avoids
   wasting energy in the proactive repair of unused paths.

   In order to cope with lossy transmissions, RPL forms Direction-
   Oriented Directed Acyclic Graphs (DODAGs) using DODAG Information
   Solicitation (DIS) and DODAG Information Object (DIO) messages.  For

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   most of the nodes, though not all, a DODAG provides multiple
   forwarding solutions towards the Root of the topology via so-called
   parents.  RPL is designed to adapt to fuzzy connectivity, whereby the
   physical topology cannot be expected to reach a stable state, with a
   lazy control that creates routes proactively but only fixes them when
   they are used by actual traffic.  It results that RPL provides
   reachability for most of the LLN nodes, most of the time, but does
   not really converge in the classical sense.  RPL provides unicast and
   multicast routing services back to RPL-Aware nodes.  A RPL-Aware Node
   will inject routes to self using Destination Advertisement Object
   (DAO) messages sent to either their parents in Storing Mode or to the
   Root indicating their parent in Non-Storing mode.  This process
   effectively forms a DODAG back to the device that is a subset of the
   DODAG to the Root with all links reversed.

   The IPv6 [RFC8200] Neighbor Discovery (IPv6 ND) Protocol (NDP) suite
   [RFC4861] [RFC4862] defined for fast media such a Ethernet, relies
   heavily on multicast operations for address discovery and duplicate
   address detection (DAD).

   "Neighbor Discovery Optimizations for 6LoWPAN networks" [RFC6775]
   (6LoWPAN ND) adapts IPv6 ND for operations over energy-constrained
   LLNs.  In particular, 6LoWPAN ND introduces a unicast host address
   registration mechanism that contributes to reduce the use of
   multicast messages that are present in the classical IPv6 ND
   protocol. 6LoWPAN ND defines a new Address Registration Option (ARO)
   that is carried in the unicast Neighbor Solicitation (NS) and
   Neighbor Advertisement (NA) messages between the 6LoWPAN Node (6LN)
   and the 6LoWPAN Router (6LR).  6LoWPAN ND also defines the Duplicate
   Address Request (DAR) and Duplicate Address Confirmation (DAC)
   messages between the 6LR and the 6LoWPAN Border Router (6LBR).  In an
   LLN, the 6LBR is the central repository of all the registered
   addresses in its domain.

   When a routing protocol such as RPL is used to maintain reachability
   within a Non-Broadcast Multi-Access (NBMA) subnet, some nodes may act
   as routers and participate to the routing operations whereas others
   may be plain hosts.  In RPL terms, a plain host that does not
   participate to the routing protocol is called a Leaf.  It must be
   noted that a 6LN could participate to RPL and inject DAO routes to
   self, but refrain from advertising DIO and get children.  In that
   case, the 6LN is still a host but not a Leaf.

   An Update to 6LoWPAN ND [I-D.ietf-6lo-rfc6775-update] defines an
   Extended ARO (EARO) with a 'R' flag to be used by a 6LN when
   registering, to indicate that this 6LN is not a router and that it
   will not handle its own reachability.  The EARO also includes a
   sequence counter called Transaction ID (TID), which maps to the Path

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   Sequence Field found in Transit Options in RPL DAO messages.  It is a
   prerequisite for this specification.  The DAR and DAC messages are
   also extended as EDAR and EDAC messages respectively.

   With this specification, a 6LN that operates as a Leaf uses the 'R'
   flag to declare itself as such and the 6LR that accepts the
   registration will inject routing information on behalf of the 6LN in
   the RPL domain.  The packet forwarding operation by the 6LR serving a
   Leaf 6LN is described in "When to use RFC 6553, 6554 and IPv6-in-
   IPv6" [I-D.ietf-roll-useofrplinfo].  This document adds the
   capability by a 6LR to advertise the IPv6 address(es) of the 6LN in
   the RPL protocol.  Examples of routing-agnostic 6LN may include
   lightly-powered sensors such as window smash sensor (alarm system),
   or the kinetically powered light switch.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

   The Terminology used in this document is consistent with and
   incorporates that described in Terms Used in Routing for Low-Power
   and Lossy Networks (LLNs).  [RFC7102].

   Other terms in use in LLNs are found in Terminology for Constrained-
   Node Networks [RFC7228].

   A glossary of classical 6LoWPAN acronyms is given in Appendix A.

   The term "byte" is used in its now customary sense as a synonym for

   "RPL", "RPL Packet Information" (RPI) and "RPL Instance", DIO, DAO
   and DIS messages are defined in the "RPL: IPv6 Routing Protocol for
   Low-Power and Lossy Networks" [RFC6550] specification.

3.  Updating RFC 6550

   This document specifies a new behavior whereby a 6LR injects DAO
   messages for unicast addresses registered through the updated 6LoWPAN
   ND [I-D.ietf-6lo-rfc6775-update] on behalf of 6LN nodes that are not

   Upon the renewal of a 6lowPAN ND registration, this specification
   changes the behavior of the 6LR as follows.  If the 'R' flag is set,
   the 6LR injects a DAO targeting the Registered Address, and refrains

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   from sending a DAR message.  the DAR/DAC exchange that refreshes the
   state in the 6LBR happens instead between the RPL Root and the 6LBR.
   In that flow, the RPL Root acts as a proxy on behalf of the 6LR upon
   the reception of the DAO propagation initiated at the 6LR.

4.  Updating RFC 6775 Update

   This document makes use of the 'R' flag in the EARO option, used by a
   6LN, when registering, to indicate that this 6LN is a Leaf, not aware
   of the RPL operation in the network, and thus does not participate to
   it.  The behavior defined in this specification whereby the 6LR that
   processes the registration advertises the Registered Address in DAO
   messages and bypasses the DAR/DAC process for the renewal of a
   registration, is only triggered by an NS(EARO) that has the 'R' flag
   set.  A RPL Leaf SHOULD set the 'R' flag.

   If the 'R' flag is not set, then the Registering Node is expected to
   be a RPL router that handles the reachability of the Registered
   Address by itself.  This document also specifies a keep-alive EDAR
   message that the RPL Root may use to maintain an existing state in
   the 6LBR upon receiving DAO messages.  The EDAR message may only act
   as a refresher and can only update the Lifetime and the TID of the
   state in the 6LBR.  A RPL router SHOULD NOT set the 'R' flag.

5.  Protocol Operations

5.1.  General Flow

   This specification enables to save the exchange of Extended Duplicate
   Address messages, EDAR and EDAC, from a 6LN all the way to the 6LBR
   across a RPL mesh, for the sole purpose of refreshing an existing
   state in the 6LBR.  Instead, the EDAR/EDAC exchange is proxied by the
   RPL Root upon a DAO message that refreshes the RPL routing state.  To
   achieve this, the lifetimes and sequence counters in 6LoWPAN ND and
   RPL are aligned.  In other words, the Path Sequence and the Path
   Lifetime in the DAO message are derived from the Transaction ID and
   the registration lifetime in the NS(EARO) message from the 6LN.

   From the perspective of the 6LN, the registration flow happens
   transparently; it is not delayed by the proxy RPL operation, so the
   device does not need to wait more whether RPL proxy operation happens
   or not.  The flows below are RPL Non-Storing Mode examples.  In
   Storing Mode, the DAO ACK may not be present, and the DAO messages
   cascade from child to parent all the way to the DODAG Root.

   On the first registration, illustrated in Figure 1, from the
   perspective of the 6LR, the Extended Duplicate Address message takes
   place as prescribed by [I-D.ietf-6lo-rfc6775-update].  When

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   successful, the flow creates a Neighbor Cache Entry (NCE) in the 6LR,
   and the 6LR injects the registered address in RPL using DAO/DAO-ACK
   exchanges all the way to the RPL DODAG Root.  The protocol does not
   carry a specific information that the Extended Duplicate Address
   messages were already exchanged, so the Root proxies them anyway.

        6LN              6LR             Root            6LBR
         |                |               |                |
         |   NS(EARO)     |               |                |
         |--------------->|               |                |
         |                | Extended DAR                   |
         |                |------------------------------->|
         |                |                                |
         |                | Extended DAC                   |
         |                |<-------------------------------|
         |   NA(EARO)     |                                |
         |<---------------|               |                |
         |                |      DAO      |                |
         |                |-------------->|                |
         |                |    DAO ACK    |                |
         |                |<--------------|                |
         |                |               | keep-alive DAR |
         |                |               |--------------->|
         |                |               | keep-alive DAC |
         |                |               |<---------------|
         |                |               |                |

                     Figure 1: First Registration Flow

   A re-registration is performed by the 6LN to maintain the NCE in the
   6LR alive before lifetime expires.  Upon a re-registration, as
   illustrated in Figure 1, the 6LR redistributes the NS(EARO) in RPL.
   This causes the RPL DODAG Root to refresh the state in the 6LBR with
   a keep-alive EDAR message.

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        6LN              6LR             Root            6LBR
         |                |               |                |
         |   NS(EARO)     |               |                |
         |--------------->|               |                |
         |   NA(EARO)     |               |                |
         |<---------------|               |                |
         |                |      DAO      |                |
         |                |-------------->|                |
         |                |    DAO ACK    |                |
         |                |<--------------|                |
         |                |               | proxy DAR      |
         |                |               |--------------->|
         |                |               | proxy DAC      |
         |                |               |<---------------|
         |                |               |                |

                     Figure 2: Next Registration Flow

   Note that any of the functions 6LR, Root and 6LBR might be collapsed
   in a single node, in which case the flow above happens internally,
   and possibly through internal API calls as opposed to messaging.

5.2.  6LN Operation

   This specification does not alter the operation of a 6LowpAN ND-
   compliant 6LN, which is expected to operate as follows:

   o  The 6LN obtains an IPv6 global address, for instance using
      autoconfiguration [RFC4862] based on a Prefix Information Option
      (PIO) [RFC4861] found in a Router Advertisement message or by some
      other means such as DHCPv6 [RFC3315].
   o  Once it has formed an address, the 6LN (re)registers its address
      periodically, within the Lifetime of the previous registration, as
      prescribed by [I-D.ietf-6lo-rfc6775-update].
   o  Upon each consecutive registration, the 6LN increases the TID
   o  The 6LN MAY register to more than one 6LR at the same time.  In
      that case, a same value of TID is used for each registration.
   o  The 6LN MAY use any of the 6LRs to which it register to forward
      its packets.

5.3.  6LR Operation

   Also as prescribed by [I-D.ietf-6lo-rfc6775-update], the 6LR
   generates a DAR message upon reception of a valid NS(EARO) message
   for the registration of a new IPv6 Address by a 6LN.  If the
   Duplicate Address exchange succeeds, then the 6LR installs a Neighbor
   Cache Entry (NCE).  If the 'R' flag was set in the EARO of the NS

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   message, and this 6LR can manage the reachability of Registered
   Address, then the 6LR sets the 'R' flag in the ARO of the response NA

   From then on, the 6LN periodically sends a new NS(EARO) to refresh
   the NCE state before the lifetime indicated in the EARO expires, with
   TID that is incremented each time till it wraps in a lollipop
   fashion.  As long as the 'R' flag is set and this router can still
   manage the reachability of Registered Address, the 6LR keeps setting
   the 'R' flag in the ARO of the response NA message, but the exchange
   of Duplicate Address messages is skipped.

   Upon a successful NS/NA(EARO) exchange: if the 'R' flag was set in
   the EARO of the NS message, then the 6LR SHOULD inject the Registered
   Address in RPL by sending a DAO message on behalf of the 6LN; else
   the 6LR SHOULD refrain from injecting the registered address into

   The DAO message advertising the Registered Address MUST be
   constructed as follows:

   o  The registered address is placed in a RPL Target Option in the DAO
      message as the Target Prefix, and the Prefix Length is set to 128
   o  the External 'E' flag in the Transit Information Option (TIO)
      associated to the Target Option is set to indicate that the 6LR
      redistributes an external target into the RPL network
   o  the Path Lifetime in the TIO is computed from the Lifetime in the
      EARO Option to adapt it to the Lifetime Units used in the RPL
      operation.  Note that if the lifetime is 0, then the 6LR generates
      a No-Path DAO message that cleans up the routes down to the
      Address of the 6LN.
   o  the Path Sequence in the TIO is set to the TID value found in the
      EARO option.
   o  Additionally, in Non-Storing Mode the 6LR indicates one of its
      global IPv6 unicast addresses as the Parent Address in the TIO.

   If a 6LR receives a valid NS(EARO) message with the 'R' flag reset
   and the 6LR was redistributing the registered address due to previous
   NS(EARO) messages with the flag set, then it MUST stop injecting the
   address.  It is up to the Registering Node to maintain the
   corresponding route from then on, either keeping it active by sending
   further DAO messages, or destroying it using a No-Path DAO.

5.4.  RPL Root Operation

   In RPL Storing Mode of Operation (MOP), the DAO message is propagated
   from child to parent all the way to the Root along the DODAG,
   populating routing state as it goes.  In Non-Storing Mode, The DAO

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   message is sent directly to the route.  Upon reception of a DAO
   message that creates or updates an existing RPL state, the Root
   notifies the 6LR using an internal API if they are collocated, or a
   proxied DAR/DAC exchange on behalf of the registering node if they
   are separated.

   In the latter case, the DAR message MUST be constructed as follows:

   o  The registered address from in the Target Option is placed in the
      Registered Address field
   o  the Owner Unique ID field is set to all ones to indicate that it
      is not provided
   o  the Registration Lifetime in the DAR message is adapted from the
      Path Lifetime in the TIO.
   o  the TID value is set to the Path Sequence in the TIO.

   Upon a status in a DAC message that is not "Success", the Root MAY
   destroy the formed paths using a No-Path DAO downwards as specified
   in [I-D.ietf-roll-efficient-npdao].

   In Non-Storing Mode, the outer IPv6 header that is used by the Root
   to transport the source routing information in data packets down the
   DODAG has the 6LR that serves the 6LN as final destination.  This
   way, when the final 6LR decapsulates the outer header, it also
   removes all the RPL artifacts from the packet.

5.5.  6LBR Operation

   Upon reception of a DAR message with the Owner Unique ID field is set
   to all ones, the 6LBR checks whether an entry exists for the and
   computes whether the TID in the DAR message is fresher than that in
   the entry as prescribed in section 4.2.1. of

   If the entry does not exist, the 6LBR does not create the entry, and
   answers with a Status "Removed" in the DAC message.

   If the entry exists but is not fresher, the 6LBR does not update the
   entry, and answers with a Status "Success" in the DAC message.

   If the entry exists and the TID in the DAR message is fresher, the
   6LBR updates the TID in the entry, and if the lifetime of the entry
   is extended by the Registration Lifetime in the DAR message, it also
   updates the lifetime of the entry.  In that case, the 6LBR replies
   with a Status "Success" in the DAC message.

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6.  Implementation Status

7.  Security Considerations

   The LLN nodes depend on the 6LBR and the RPL participants 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.
   This trust model could be at a minimum based on a Layer-2 access
   control, or could provide role validation as well.  This is a generic
   6LoWPAN requirement, see Req5.1 in Appendix of

   The keep-alive EDAR message does not carry a valid Registration
   Unique ID [I-D.ietf-6lo-rfc6775-update] and it cannot be used to
   create a binding state in the 6LBR.  The 6LBR MUST NOT create an
   entry based on a keep-alive EDAR that does not match an existing
   entry.  All it can do is refresh the lifetime and the TID of an
   existing entry.

8.  IANA Considerations

   This specification has no requirement on IANA.

9.  Acknowledgments

   The author wishes to thank Michael Richardson and Georgios
   Papadopoulos for their early reviews of and contributions to this

10.  References

10.1.  Normative References

              Thubert, P., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "An Update to 6LoWPAN ND", draft-ietf-6lo-
              rfc6775-update-13 (work in progress), February 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [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,

   [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,

   [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,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

10.2.  Informative References

              Thubert, P., Sarikaya, B., and M. Sethi, "Address
              Protected Neighbor Discovery for Low-power and Lossy
              Networks", draft-ietf-6lo-ap-nd-05 (work in progress),
              January 2018.

              Jadhav, R., Sahoo, R., and Z. Cao, "No-Path DAO
              modifications", draft-ietf-roll-efficient-npdao-01 (work
              in progress), October 2017.

              Robles, I., Richardson, M., and P. Thubert, "When to use
              RFC 6553, 6554 and IPv6-in-IPv6", draft-ietf-roll-
              useofrplinfo-21 (work in progress), February 2018.

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              IEEE standard for Information Technology, "IEEE Standard
              for Local and metropolitan area networks-- Part 15.4: Low-
              Rate Wireless Personal Area Networks (LR-WPANs)".

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <>.

   [RFC6687]  Tripathi, J., Ed., de Oliveira, J., Ed., and JP. Vasseur,
              Ed., "Performance Evaluation of the Routing Protocol for
              Low-Power and Lossy Networks (RPL)", RFC 6687,
              DOI 10.17487/RFC6687, October 2012,

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

Appendix A.  Subset of a 6LoWPAN Glossary

   This document often uses the followng acronyms:

   6BBR: 6LoWPAN Backbone Router (proxy for the registration)
   6LBR: 6LoWPAN Border Router (authoritative on DAD)
   6LN:  6LoWPAN Node
   6LR:  6LoWPAN Router (relay to the registration process)
   6CIO: Capability Indication Option
   (E)ARO:  (Extended) Address Registration Option
   DAD:  Duplicate Address Detection
   LLN:  Low Power Lossy Network (a typical IoT network)
   NCE:  Neighbor Cache Entry
   RUID: Registration Unique ID
   TID:  Transaction ID (a sequence counter in the EARO)

Author's Address

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   Pascal Thubert (editor)
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   MOUGINS - Sophia Antipolis  06254

   Phone: +33 497 23 26 34

Thubert                  Expires August 27, 2018               [Page 13]