3.5. Rank Properties

The Rank of a node is a scalar representation of the location of that node within a DODAG Version. The Rank is used to avoid and detect loops and, as such, must demonstrate certain properties. The exact calculation of the Rank is left to the Objective Function. Even though the specific computation of the Rank is left to the Objective Function, the Rank must implement generic properties regardless of the Objective Function.

In particular, the Rank of the nodes must monotonically decrease as the DODAG Version is followed towards the DODAG destination. In that regard, the Rank can be considered a scalar representation of the location or radius of a node within a DODAG Version.

The details of how the Objective Function computes Rank are out of scope for this specification, although that computation may depend, for example, on parents, link metrics, node metrics, and the node configuration and policies. See Section 14 for more information.

The Rank is not a path cost, although its value can be derived from and influenced by path metrics. The Rank has properties of its own that are not necessarily those of all metrics:

• Type: The Rank is an abstract numeric value.

• Function: The Rank is the expression of a relative position within a DODAG Version with regard to neighbors, and it is not necessarily a good indication or a proper expression of a distance or a path cost to the root.

• Stability: The stability of the Rank determines the stability of the routing topology. Some dampening or filtering is RECOMMENDED to keep the topology stable; thus, the Rank does not necessarily change as fast as some link or node metrics would. A new DODAG Version would be a good opportunity to reconcile the discrepancies that might form over time between metrics and Ranks within a DODAG Version.

• Properties: The Rank is incremented in a strictly monotonic fashion, and it can be used to validate a progression from or towards the root. A metric, like bandwidth or jitter, does not necessarily exhibit this property.

• Abstract: The Rank does not have a physical unit, but rather a range of increment per hop, where the assignment of each increment is to be determined by the Objective Function.

The Rank value feeds into DODAG parent selection, according to the RPL loop-avoidance strategy. Once a parent has been added, and a Rank value for the node within the DODAG has been advertised, the node's further options with regard to DODAG parent selection and movement within the DODAG are restricted in favor of loop avoidance.