Controlling Secure Network Enrollment in RPL networks

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Table of Contents

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 used to describe the enrollment process. However, the term Join Proxy is retained with it's meaning from [I-D.ietf-6tisch-minimal-security].¶

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.¶

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.¶

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¶

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.¶

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.¶

6. Acknowledgements

This has been reviewed by Pascal Thubert and Thomas Wattenye.¶

Appendix A. Change history

version 00.¶

Authors' Addresses