3.5.2. Rank Relationships

Rank computations maintain the following properties for any nodes M and N that are neighbors in the LLN:

• DAGRank(M) is less than DAGRank(N):

  In this case, the position of M is closer to the DODAG root than the position of N. Node M may safely be a DODAG parent for Node N without risk of creating a loop. Further, for a Node N, all parents in the DODAG parent set must be of a Rank less than DAGRank(N). In other words, the Rank presented by a Node N MUST be greater than that presented by any of its parents.

• DAGRank(M) equals DAGRank(N):

  In this case, the positions of M and N within the DODAG and with respect to the DODAG root are similar or identical. Routing through a node with equal Rank may cause a routing loop (i.e., if that node chooses to route through a node with equal Rank as well).

• DAGRank(M) is greater than DAGRank(N):

  In this case, the position of M is farther from the DODAG root than the position of N. Further, Node M may in fact be in the sub-DODAG of Node N. If Node N selects Node M as DODAG parent, there is a risk of creating a loop.

As an example, the Rank could be computed in such a way so as to closely track ETX (expected transmission count, a fairly common routing metric used in LLN and defined in [RFC6551]) when the metric that an Objective Function minimizes is ETX, or latency, or in a more complicated way as appropriate to the Objective Function being used within the DODAG.