Controlling Secure Network Enrollment in RPL networks
draft-ietf-roll-enrollment-priority-14

ROLL Working Group                                         M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                            R. A. Jadhav
Expires: 17 May 2026                                         Huawei Tech
                                                              P. Thubert
                                                           Cisco Systems
                                                             K. Iwanicki
                                                    University of Warsaw
                                                        13 November 2025

         Controlling Secure Network Enrollment in RPL networks
                 draft-ietf-roll-enrollment-priority-14

Abstract

   [RFC9032] defines a method by which a potential [RFC9031] enrollment
   proxy can announce itself as available for new Pledges to enroll on a
   network.  The announcement includes a priority for enrollment.  This
   document provides a mechanism by which a Routing Protocol for Low-
   Power and Lossy Networks (RPL) Root can globally disable enrollment
   announcements or adjust the base priority for enrollment operations.

Status of This Memo

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   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 17 May 2026.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation and Overview . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Definition . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Option Format . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Option Processing . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Upwards Compatibility . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   [RFC7554] describes the use of the Time-Slotted Channel Hopping
   (TSCH) mode of [ieee802154].  [RFC9031] and [RFC9032] describe
   mechanisms by which a new node (the "Pledge") can use a nearby router
   as a Join Proxy.  [RFC9032] describes an extension to the 802.15.4
   Enhanced Beacon that is used by a Join Proxy to announce its
   existence such that Pledges can find them.

1.1.  Motivation and Overview

   It has become clear that not every routing member of the mesh ought
   to announce itself as a _Join Proxy_. There are a variety of local
   reasons for which a 6LowPAN Router (6LR) might not want to provide
   the _Join Proxy_ function.  They include low available battery power,
   already high committed network bandwidth, and available free memory
   for Neighbor Cache Entry (NCE) slots.  (An NCE entry is needed in
   order to maintain communication with the Pledge nodes trying to
   enroll)

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   There are other situations where the operator of the network would
   like to selectively enable or disable the enrollment process in a
   specific Destination Oriented Directed Acyclic Graph (DODAG).  In
   particular, as the enrollment process involves permitting unencrypted
   traffic into the best effort part of a network, it would be better to
   have the enrollment process off when no new nodes are expected.

   This document describes a Routing Protocol for Low-Power and Lossy
   Networks (RPL) Destination Information Object (DIO) option that can
   be used to set a minimum enrollment priority.  The minimum priority
   expresses the inability of the RPL DODAG globally to accept new
   joins.  It may derive from multiple constraining factors, for
   instance, the size of the DODAG, the occupancy of the bandwidth at
   the DODAG Root, the memory capacity at the Root, or an administrative
   decision.  Each potential _Join Proxy_ utilizes this value as a base
   on which to add values relating to local conditions, such as its Rank
   and number of pending joins.  As explained in [RFC9032], higher
   values decrease the likelihood of an unenrolled node sending
   enrollment traffic via this _Join Proxy_. In particular, by setting
   the minimum enrollment priority to the maximum value allowed, a
   network operator can globally disable all new enrollment traffic.

   Moreover, when a RPL domain is composed of multiple DODAGs, a node at
   the edge of more than one such DODAG may not only join any of the
   DODAGs but also move between them in order to keep their relative
   sizes balanced.  For this, the approximate knowledge of the size of
   the DODAGs is also an essential metric.  Depending on the network
   policy, the size of the DODAG may or may not affect the minimum
   enrollment priority.  Therefore, since making one proportional to the
   other would be limiting their value, the current size of the DODAG is
   advertised separately in the new option.

   Updates to the option propagate through the network according to the
   trickle algorithm.  The contents of the option are generated at the
   DODAG Root and do not change at any hop.  If the contents represent
   an update that is considered important (e.g., quickly disabling any
   enrollments), the option can trigger trickle timer resets at the
   nodes to speed up its propagation.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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   The term 6LR means 6LowPAN Router, and is defined in [RFC6606].  It
   refers to a router that forwards packets in a 6LowPAN network.

   The terms DAO, DODAG, DODAG root, DIO, trickle timer are from
   [RFC6550].  The lollipop counter function comes from [RFC6550],
   Section 7.2.

   The term (1)"Join" has been used in documents such as [RFC9031] to
   denote the activity of a new node authenticating itself to the
   network to obtain authorization to become a member of the network.

   In the context of the [RFC6550] RPL protocol, the term (2)"Join" has
   an alternative meaning: that of a node (already authenticated to the
   network, and already authorized to be a member of the network),
   deciding which part of the RPL DODAG to attach to.  This term "Join"
   has to do with preferred parent selection processes.

   In order to avoid the ambiguity of this term, this document refers to
   the process (1)"Join" as enrollment, leaving the term "Join" to mean
   (2)"Join".  The term "onboarding" (or "IoT Onboarding") is
   increasingly used to describe what is now called (1)Join in other
   documents, and is called enrollment in this document.  However, the
   term _Join Proxy_ is retained with its (1)"Join" meaning from
   [RFC9031].

3.  Protocol Definition

   This document uses the extensions mechanism designed into [RFC6550].
   No mechanism is needed to enable it.

3.1.  Option Format

   The following option is defined for transmission in DIOs issued by
   the DODAG Root to be propagated within the DODAG.

       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 = TBD01  |Opt Length = 4 |Version Number |T| Min Priority|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Exp  |DODAGSz|
      +-+-+-+-+-+-+-+-+

   Type  To be assigned by IANA.

   Version Number  An 8-bit unsigned integer set by the DODAG root and

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      denoting the version number of the contents of the option.  The
      version number is interpreted as a lollipop counter (see
      Section 7.2 of [RFC6550]).

   T  A bit indicating whether the particular version of the option is
      important in that adopting its contents should trigger a trickle
      timer reset at the node.

   Min Priority  A 7-bit field providing a base value for the Enhanced
      Beacon Join priority.  A value of 0x7f (127) disables the _Join
      Proxy_ function entirely.

   Exp  A 4-bit unsigned integer indicating the power of 2 that defines
      the unit of the DODAG Size, such that (unit = 2^Exp).

   DODAGSz  A 4-bit unsigned integer expressing the size of the DODAG in
      units that depend on the Exp field.

   The DODAG Size is calculated as (DODAGSz * 2^Exp).

   The DODAG Size can be measured by the Root based on the DAO activity.
   In such a case, it represents the number of routes not the number of
   nodes, and can thus be used to infer the load only in a network where
   each node advertises roughly the same number of addresses and
   generates roughly the same amount of traffic.

   As the DODAG Size is always a multiple of a power of 2, when the
   actual size falls between two such values, the DODAG Root is to
   always round up.

   Future work such as [I-D.ietf-roll-capabilities] will enable
   collection of capabilities such as this one in reports to the DODAG
   Root.

   In any case, the DODAG Size may slightly change between a DIO and the
   next, so the value transmitted is considered as an approximation.

3.2.  Option Processing

   The contents of the option MUST be generated by the DODAG Root.  A
   6LR MUST NOT change them when propagating the option.

   Whenever the DODAG root changes the values of Min Priority or DODAG
   Size in the option, it MUST also increment the value of Version
   Number.  Moreover, if the change is considered important (i.e., it is
   expected to propagate in the DODAG quickly), the DODAG Root SHOULD
   also set the T bit to 1; otherwise, it MUST set the bit to 0.

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   Upon receiving the option, a 6LR first checks the value of the
   Version Number field in the option, _vr_, versus the value of the
   Version Number it has last adopted locally, _vl_.

   *  If _vl_ is greater than _vr_ (in the lollipop counter order), then
      the 6LR MUST ignore the received option.

   *  Otherwise, the 6LR MUST adopt the contents of the option (i.e.,
      the values of Version Number, Min Priority, DODAG Size, and the T
      bit) as its local ones.  Moreover, if _vl_ was smaller than _vr_
      (in the lollipop counter order) and the T bit in the received
      option was set, then the 6LR MUST reset its DIO trickle timer.

   A 6LR, which would otherwise be willing to act as a _Join Proxy_,
   will examine the locally adopted value of Min Priority and to that
   number add any additional local consideration (such as upstream
   congestion, number of NCE slots available, etc.).

   The maximum resulting value any 6LR can obtain this way is 0x7f.

   The resulting priority, if less than 0x7f, should enable the _Join
   Proxy_ function.

3.3.  Upwards Compatibility

   A 6LR that did not support this option would not act on it or
   propagate it in its DIO messages.  In effect, the 6LR's children and
   grandchildren nodes could not receive any telemetry.  Therefore, 6LRs
   that support this option but do not receive it via any path SHOULD
   assume a default value of 0x40 as their base value for the Enhanced
   Beacon Join Priority.

   A 6LR downstream of a 6LR where there was such an interruption in the
   telemetry could err in two directions:

   *  If the value implied by the base value of 0x40 was too low, then
      the 6LR might continue to attract enrollment traffic when none
      should have been collected.  This is a stressor for the network,
      but this would also be what would occur without this option at
      all.

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   *  If the value implied by the base value of 0x40 was too high, then
      the 6LR might deflect enrollment traffic to other parts of the
      DODAG, possibly refusing any enrollment traffic at all.  In order
      for this to happen, some significant congestion must be seen in
      the sub-DODAG where the implied 0x40 was introduced.  The 0x40 is
      only the half-way point, so if such an amount of congestion was
      present, then this sub-DODAG of the DODAG simply winds up being
      more cautious than it needed to be.

   It is possible that the temporal alternation of the above two
   situations might introduce cycles of accepting and then rejecting
   enrollment traffic.  This is something an operator should consider if
   they incrementally deploy this option to an existing Low-power/Lossy-
   Network (LLN).  In addition, an operator would be unable to turn off
   enrollment traffic by sending a maximum value enrollment priority to
   the sub-DODAG.  This situation is unfortunate, but without this
   option, the situation would occur all over the DODAG, rather than
   just in the sub-DODAG that the option did not reach.

4.  Security Considerations

   As per [RFC7416], RPL control frames either run over a secured layer
   2 or use the [RFC6550] Secure DIO methods at layer 3.  This option
   can be placed into either a "clear" (layer-2 secured) DIO or a
   layer-3 Secure DIO.

   In most deployments involving wireless technology, layer 2 is always
   encrypted using a layer-2 specific technology, and so privacy of this
   option is available.

   However, a malicious node that was part of the RPL control plane
   (i.e., had been enrolled into the layer-2 security) would be able to
   see the values of this option and, based upon the observed minimal
   enrollment priority, could signal a confederate that it was a good
   time to send malicious join traffic.

   What is more, such a malicious node, being already part of the RPL
   control plane, could also send DIOs with a different minimal
   enrollment priority, which would cause downstream mesh routers to
   change their _Join Proxy_ behavior: lower minimal priorities would
   cause downstream nodes to accept more Pledges than the network was
   expecting; higher minimal priorities could cause the enrollment
   process to stall.

   The use of layer-2 or layer-3 security for RPL control messages
   prevents the two aforementioned attacks by non-participating nodes by
   preventing malicious nodes from becoming part of the control plane.

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   Nevertheless, a node that is attacked and has malware placed on it
   creates vulnerabilities in the same way such an attack on any node
   involved in Internet routing protocol does.  The rekeying provisions
   of [RFC9031] exist to permit an operator to remove such nodes from
   the network.

5.  Privacy Considerations

   There are no new privacy issues caused by this extension.

6.  IANA Considerations

   Allocate a new number TBD01 from Registry RPL Control Message
   Options.  This entry should be called Minimum Enrollment Priority.

7.  Acknowledgements

   This has been reviewed by Charlie Perkins, Rifaat Shehk-Yusek, Dave
   Thaler, and Thomas Watteyne.

   Huimin She contributed text about expressing the DODAG size.

8.  References

8.1.  Normative References

   [ieee802154]
              IEEE standard for Information Technology, "IEEE Std.
              802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
              and Physical Layer (PHY) Specifications for Low-Rate
              Wireless Personal Area Networks", n.d.,
              <http://standards.ieee.org/findstds/
              standard/802.15.4-2015.html>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/rfc/rfc6550>.

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   [RFC7416]  Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A.,
              and M. Richardson, Ed., "A Security Threat Analysis for
              the Routing Protocol for Low-Power and Lossy Networks
              (RPLs)", RFC 7416, DOI 10.17487/RFC7416, January 2015,
              <https://www.rfc-editor.org/rfc/rfc7416>.

   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015,
              <https://www.rfc-editor.org/rfc/rfc7554>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC9031]  Vučinić, M., Ed., Simon, J., Pister, K., and M.
              Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH",
              RFC 9031, DOI 10.17487/RFC9031, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9031>.

   [RFC9032]  Dujovne, D., Ed. and M. Richardson, "Encapsulation of
              6TiSCH Join and Enrollment Information Elements",
              RFC 9032, DOI 10.17487/RFC9032, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9032>.

8.2.  Informative References

   [I-D.ietf-roll-capabilities]
              Jadhav, R., Thubert, P., Richardson, M., and R. N. Sahoo,
              "RPL Capabilities", Work in Progress, Internet-Draft,
              draft-ietf-roll-capabilities-09, 9 November 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-roll-
              capabilities-09>.

   [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Routing",
              RFC 6606, DOI 10.17487/RFC6606, May 2012,
              <https://www.rfc-editor.org/rfc/rfc6606>.

Authors' Addresses

   Michael Richardson
   Sandelman Software Works
   Email: mcr+ietf@sandelman.ca

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   Rahul Arvind Jadhav
   Huawei Tech
   Email: rahul.ietf@gmail.com

   Pascal Thubert
   Cisco Systems
   Email: pthubert@cisco.com

   Konrad Iwanicki
   University of Warsaw
   Email: iwanicki@mimuw.edu.pl

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