Controlling Secure Network Enrollment in RPL networks

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. 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. This typically involves a cryptographic authentication protocol in which a network credential is provided. In the context of the 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 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 ) among all nodes in the subtree of a congested link. This document describes an RPL DIO option that can be used to announce a minimum enrollment priority. Each potential Join Proxy would this value as a base on which to add values relating to local conditions. As explained in , 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.

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 following option is defined to transmission in the DIO issued by the DODAG root. It may also be added by a router on part of the sub-tree as a result of some (out of scope for this document) management function. 6LRs that see this DIO Option SHOULD increment their minimum enrollment priority if they observe congestion on the channel used for enrollment traffic. The exact mechanism is a local decision, and may be the subject for future work. 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.

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. 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 . It does not need any mechanism to enable it. Future work like will enable collection of capabilities such as this one in reports to the DODAG root.

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. 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.

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 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 exist to permit an operator to remove such nodes from the network easily.

There are no new privacy issues caused by this extension.

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

This has been reviewed by Pascal Thubert and Thomas Wattenye.