ROLL Working Group                                         M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                             R.A. Jadhav
Expires: 11 August 2021                                      Huawei Tech
                                                              P. Thubert
                                                                  H. She
                                                           Cisco Systems
                                                         7 February 2021

         Controlling Secure Network Enrollment in RPL networks


   [I-D.ietf-6tisch-enrollment-enhanced-beacon] defines a method by
   which a potential [I-D.ietf-6tisch-minimal-security] 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

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 August 2021.

Copyright Notice

   Copyright (c) 2021 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 (
   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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Protocol Definition . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Upwards compatibility . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Change history . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [RFC7554] describes the use of the time-slotted channel hopping
   (TSCH) mode of [ieee802154].  [I-D.ietf-6tisch-minimal-security] and
   [I-D.ietf-6tisch-dtsecurity-secure-join] describe mechanisms by which
   a new node (the "pledge)" can use a friendly router as a Join Proxy.
   [I-D.ietf-6tisch-enrollment-enhanced-beacon] 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.

   The term (1)"Join" has been used in documents like
   [I-D.ietf-6tisch-minimal-security] 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.  This typically
   involves a cryptographic authentication protocol in which a network
   credential is provided.

   In the context of the [RFC6550] RPL protocol, the term (2)"Join" has
   an alternate 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 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 sometimes

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   used to describe the enrollment process.  However, the term _Join
   Proxy_ is retained with it's meaning from

   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 also total available memory available for
   Neighbor Cache Entry slots.

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

   As the enrollment process involves permitting unencrypted traffic
   into the best effort part of a (TSCH) network, it would be better to
   have the enrollment process off when no new nodes are expected.

   A network operator might also be able to recognize when certain parts
   of the network are overloaded and can not accomodate additional
   enrollment traffic, and it would like to adjust the enrollment
   priority (the proxy priority field of
   [I-D.ietf-6tisch-enrollment-enhanced-beacon]) among all nodes in the
   subtree of a congested link.

   This document describes a RPL DIO option that can be used to announce
   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 constaining 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 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.
   This is why the current size of the DODAG is advertised separately in
   the new option.

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   As explained in [I-D.ietf-6tisch-enrollment-enhanced-beacon], higher
   values decrease the likelyhood of an unenrolled node sending
   enrollment traffic via this path.

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

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

2.  Protocol Definition

   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 an 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 approxmation.

   With this specification, the following option is defined for
   transmission in the DIO issued by the DODAG root and it MUST be
   propagated down the DODAG without modification.

   A 6LR which would otherwise be willing to act as a _Join Proxy_, will
   examine the minimum priority field, and to that number, add any
   additional local consideration (such as upstream congestion).

   The Enrollment Priority can only be increased by each 6LR in value,
   to the maximum value of 0x7f.

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

       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        |Opt Length = 3 |  exp  |     DODAG_Size        |
      |R| min priority|

   Type  To be assigned by IANA

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   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).

   min.priority  a 7 bit field which provides a base value for the
      Enhanced Beacon Join priority.  A value of 0x7f (127) disables the
      _Join Proxy_ function entirely.

   R  a reserved bit that SHOULD be set to 0 by senders, and MUST be
      ignored by receivers.  This reserved bit SHOULD be copied to
      options created.

   This document uses the extensions mechanism designed into [RFC6550].
   It does not need any mechanism to enable it.

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

2.1.  Upwards compatibility

   A 6LR which did not support this option would not act on it, or copy
   it into it's DIO messages.  Children and grandchildren nodes would
   therefore not receive any telemetry via that path, and need to assume
   a default value.

   6LRs that support this option, but whose parent does not send it
   SHOULD assume a value of 0x40 as their base value.  The nodes then
   adjust this base value based upon their observed congestion, emitting
   their adjusted DIO value to their children.

   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.

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   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.

3.  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_ behaviour.

   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 [I-D.ietf-6tisch-minimal-security] exist to
   permit an operator to remove such nodes from the network easily.

4.  Privacy Considerations

   There are no new privacy issues caused by this extension.

5.  IANA Considerations

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

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6.  Acknowledgements

   This has been reviewed by Pascal Thubert and Thomas Wattenye.

7.  References

7.1.  Normative References

              Dujovne, D. and M. Richardson, "IEEE 802.15.4 Information
              Element encapsulation of 6TiSCH Join and Enrollment
              Information", Work in Progress, Internet-Draft, draft-
              ietf-6tisch-enrollment-enhanced-beacon-14, 21 February
              2020, <

              Vucinic, M., Simon, J., Pister, K., and M. Richardson,
              "Constrained Join Protocol (CoJP) for 6TiSCH", Work in
              Progress, Internet-Draft, draft-ietf-6tisch-minimal-
              security-15, 10 December 2019, <

              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.,

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

   [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,

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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,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

7.2.  Informative References

              Richardson, M., "6tisch Secure Join protocol", Work in
              Progress, Internet-Draft, draft-ietf-6tisch-dtsecurity-
              secure-join-01, 25 February 2017, <

              Jadhav, R., Thubert, P., Richardson, M., and R. Sahoo,
              "RPL Capabilities", Work in Progress, Internet-Draft,
              draft-ietf-roll-capabilities-07, 17 September 2020,

Appendix A.  Change history

   version 00.

Authors' Addresses

   Michael Richardson
   Sandelman Software Works


   Rahul Arvind Jadhav
   Huawei Tech


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   Pascal Thubert
   Cisco Systems


   Huimin She
   Cisco Systems


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