Controlling Secure Network Enrollment in RPL networks

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 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.)¶

There are other situations where the operator of the network would like to selectively enable or disable the enrollment process in a specific 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 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, 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.¶

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]).¶

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 size of the DODAG is computed as (DODAGSz * 2^Exp).¶

The size of the DODAG 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. Future work like [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 MUST be 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.¶

• 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 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. 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.¶

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 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 the situation would occur all over the DODAG, rather than just in the sub-DODAG that the option did not reach.¶