ROLL Working Group M. Richardson
Internet-Draft Sandelman Software Works
Intended status: Standards Track R. A. Jadhav
Expires: 4 September 2025 Huawei Tech
P. Thubert
H. She
Cisco Systems
K. Iwanicki
University of Warsaw
3 March 2025
Controlling Secure Network Enrollment in RPL networks
draft-ietf-roll-enrollment-priority-12
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 RPL DODAG Root can globally
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
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This Internet-Draft will expire on 4 September 2025.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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.
Please review these documents carefully, as they describe your rights
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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 . . . . . . . . . . . . . . . . . . . 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 little free memory for
Neighbor Cache Entry (NCE) slots. (An NCE entry is needed in order
to maintain communication with the Pledge.)
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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 (lack of) willingness by 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 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 enrollment in other
documents. 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
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]).
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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.
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_.
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* 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.
* 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.
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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
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.
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.
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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 Thomas Watteyne, Rifaat Shehk-Yusek, Dave
Thaler,
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>.
[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>.
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[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] VuÄinić, 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
Rahul Arvind Jadhav
Huawei Tech
Email: rahul.ietf@gmail.com
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Pascal Thubert
Cisco Systems
Email: pthubert@cisco.com
Huimin She
Cisco Systems
Email: hushe@cisco.com
Konrad Iwanicki
University of Warsaw
Email: iwanicki@mimuw.edu.pl
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