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Internet ROLL Working Group Internet-Draft defines a method by which a potential 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.

Introduction describes the use of the time-slotted channel hopping (TSCH) mode of . and describe mechanisms by which a new node (the "pledge") can use a friendly router as a Join Proxy. 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.

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. 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 , 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.

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 when, and only when, they appear in all capitals, as shown here. The term (1)"Join" has been used in documents like 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 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 .

Protocol Definition This document uses the extensions mechanism designed into . No mechanism is needed to enable it.

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

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 ).

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).

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 will enable collection of capabilities such as this one in reports to the DODAG root.

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.

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.

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.

Security Considerations As per , RPL control frames either run over a secured layer 2, or use the 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 exist to permit an operator to remove such nodes from the network easily.

Privacy Considerations There are no new privacy issues caused by this extension.

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

Acknowledgements This has been reviewed by Konrad Iwanicki and Thomas Watteyne.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the environment, problem statement, and goals for using the Time-Slotted Channel Hopping (TSCH) Medium Access Control (MAC) protocol of IEEE 802.14.4e in the context of Low-Power and Lossy Networks (LLNs). The set of goals enumerated in this document form an initial set only. Low-Power and Lossy Networks (LLNs) are a class of network in which both the routers and their interconnect are constrained. LLN routers typically operate with constraints on processing power, memory, and energy (battery power). Their interconnects are characterized by high loss rates, low data rates, and instability. LLNs are comprised of anything from a few dozen to thousands of routers. Supported traffic flows include point-to-point (between devices inside the LLN), point-to-multipoint (from a central control point to a subset of devices inside the LLN), and multipoint-to-point (from devices inside the LLN towards a central control point). This document specifies the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL), which provides a mechanism whereby multipoint-to-point traffic from devices inside the LLN towards a central control point as well as point-to-multipoint traffic from the central control point to the devices inside the LLN are supported. Support for point-to-point traffic is also available. [STANDARDS-TRACK] In the Time-Slotted Channel Hopping (TSCH) mode of IEEE Std 802.15.4, opportunities for broadcasts are limited to specific times and specific channels. Routers in a TSCH network transmit Enhanced Beacon (EB) frames to announce the presence of the network. This document provides a mechanism by which additional information critical for new nodes (pledges) and long-sleeping nodes may be carried within the EB in order to conserve use of broadcast opportunities. This document describes the minimal framework required for a new device, called a "pledge", to securely join a 6TiSCH (IPv6 over the Time-Slotted Channel Hopping mode of IEEE 802.15.4) network. The framework requires that the pledge and the JRC (Join Registrar/Coordinator, a central entity), share a symmetric key. How this key is provisioned is out of scope of this document. Through a single CoAP (Constrained Application Protocol) request-response exchange secured by OSCORE (Object Security for Constrained RESTful Environments), the pledge requests admission into the network, and the JRC configures it with link-layer keying material and other parameters. The JRC may at any time update the parameters through another request-response exchange secured by OSCORE. This specification defines the Constrained Join Protocol and its CBOR (Concise Binary Object Representation) data structures, and it describes how to configure the rest of the 6TiSCH communication stack for this join process to occur in a secure manner. Additional security mechanisms may be added on top of this minimal framework. n.d. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document presents a security threat analysis for the Routing Protocol for Low-Power and Lossy Networks (RPLs). The development builds upon previous work on routing security and adapts the assessments to the issues and constraints specific to low-power and lossy networks. A systematic approach is used in defining and evaluating the security threats. Applicable countermeasures are application specific and are addressed in relevant applicability statements. Informative References Huawei Cisco Systems, Inc Sandelman Software Works Juniper This draft enables the discovery, advertisement and query of capabilities for RPL nodes.

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