Coordinates

A node’s coordinates describe the location of the node on the spanning tree. In effect, the coordinates are the path from the root node down to a given node, with each number representing the downward port number at each hop.

For example, coordinates [1 4 2 4] describe the following path: starting via the root node’s port 1, followed by the next node’s port 4, followed by the next node’s port 2 and then, finally, the destination node’s parent’s port 4.

The coordinates of the node are calculated by taking the chosen parent’s last root announcement and concatenating all of the Destination port number values in order.

With that in mind, a root node, which has no chosen parent, will have a zero-length set of coordinates [] and a node’s coordinates will change each time it either selects a new parent, or receives a root announcement update from the parent with a different path described in the signatures.