Controlling Secure Network Enrollment in RPL networks
draft-ietf-roll-enrollment-priority-09

ROLL Working Group                                         M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                            R. A. Jadhav
Expires: 17 November 2023                                    Huawei Tech
                                                              P. Thubert
                                                                  H. She
                                                           Cisco Systems
                                                             K. Iwanicki
                                                             16 May 2023

         Controlling Secure Network Enrollment in RPL networks
                 draft-ietf-roll-enrollment-priority-09

Abstract

   [RFC9032] defines a method by which a potential [RFC9031] enrollment
   proxy can announce itself as a available for new Pledges to enroll on
   a network.  The announcement includes a priority for enrollment.
   This document provides a mechanism by which a RPL DODAG root can
   disable enrollment announcements, or adjust the base priority for
   enrollment operations.

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
   Task Force (IETF).  Note that other groups may also distribute
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 17 November 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.

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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

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  . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Change history . . . . . . . . . . . . . . . . . . .   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 friendly
   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 by which a 6LR might not want to provide the _Join Proxy_
   function.  They include available battery power, already committed
   network bandwidth, and total available memory available for Neighbor
   Cache Entry (NCE) slots.  An NCE entry is needed in order to maintain
   communication with the pledge.

   There are other situations where the operator of the network would
   like to selectively enable or disable the enrollment process in a
   particular DODAG.

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   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 RPL DIO option that can be used to set a
   minimum enrollment priority.  The minimum priority expresses the
   (lack of) willingness by the RPL DODAG globally to accept new joins.

   It may derive from multiple constraining factors, e.g., the size of
   the DODAG, the occupancy of the bandwidth at the Root, the memory
   capacity at the DODAG Root, or an administrative decision.

   Each potential _Join Proxy_ would utilize this value as a base on
   which to add values relating to local conditions such as its Rank and
   number of pending joins, which would degrade even further the
   willingness to take more joins.

   When a RPL domain is composed of multiple DODAGs, nodes at the edge
   of 2 DODAGs may not only join either DODAG but also move from one to
   the other in order to keep their relative sizes balanced.  For this,
   the approximate knowledge of size of the DODAG is an essential
   metric.  Depending on the network policy, the size of the DODAG may
   or may not affect the minimum enrollment priority.  It would be
   limiting its value to enforce that one is proportional to the other.
   The current size of the DODAG is therefore also advertised in the new
   option.

   As explained in [RFC9032], higher values decrease the likelihood of
   an unenrolled node sending enrollment traffic via this path.

   A network operator can set this value to the maximum value allowed,
   effectively disabling all new enrollment traffic.

   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 (1)"Join" has been used in documents like [RFC9031] to
   denote the activity of a new node authenticating itself to the
   network in order 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 authenticating 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 was called enrollment in other documents.
   However, the term _Join Proxy_ is retained with its 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  |     DODAG_Size        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type  to be assigned by IANA.

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

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      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).

   DODAG_Size  a 12-bit unsigned integer expressing the size of the
      DODAG in units that depend on the exp field.  The size of the
      DODAG is computed as (DAG_Size*2^exp).

   The size of the DODAG is measured by the Root based one the DAO
   activity.  It represents a number of routes not a number of nodes,
   and can only be used to infer a load in a homogeneous network where
   each node advertises the same number of addresses and generates
   roughly the same amount of traffic.  The size may slightly change
   between a DIO and the next, so the value transmitted MUST be
   considered as an approximation.

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

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.

   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.

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   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 which 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 via that path.
   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 an interruption in the
   telemetry could err in two directions:

   *  if the value implied by the base value of 0x40 was too low, then a
      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.

   *  if the value implied by the base value of 0x40 was too high, then
      a 6LR might deflect enrollment traffic to other parts of the DODAG
      tree, possibly refusing any enrollment traffic at all.  In order
      for this to happen, some significant congestion must be seen in
      the sub-tree 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-tree 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
   when they incrementally deploy this option to an existing LLN.  In
   addition, an operator would be unable to turn off enrollment traffic
   by sending a maximum value enrollment priority to the sub-tree.  This
   situation is unfortunate, but without this option, the the situation
   would occur all over the DODAG, rather than just in the sub-tree
   where the option was omitted.

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4.  Security Considerations

   As per [RFC7416], RPL control frames either run over a secured layer
   2, or use the [RFC6550] Secure DIO methods.  This option can be
   placed into either a "clear" (layer-2 secured) DIO, or a layer-3
   Secure DIO.  As such this option will have both integrity and
   confidentiality mechanisms applied to it.

   A malicious node (that was part of the RPL control plane) could see
   these options and could, based upon the observed minimal enrollment
   priority signal a confederate that it was a good time to send
   malicious join traffic.

   Such as 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, and higher minimal priorities
   cause the enrollment process to stall.

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

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 Konrad Iwanicki and Thomas Watteyne.

8.  References

8.1.  Normative References

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   [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/info/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/info/rfc6550>.

   [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/info/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/info/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/info/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/info/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/info/rfc9032>.

8.2.  Informative References

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   [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>.

Appendix A.  Change history

   version 00.

Authors' Addresses

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

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

   Pascal Thubert
   Cisco Systems
   Email: pthubert@cisco.com

   Huimin She
   Cisco Systems
   Email: hushe@cisco.com

   Konrad Iwanicki
   Email: iwanicki@mimuw.edu.pl

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