Design, implementation and evaluation of an energy efficient RPL routing metric

RPL is a proactive distance-vector (see Chapter 1) IPv6 routing protocol which is designed for networks with significantly higher packet loss rates than traditional IP routing protocols.
RPL is optimized for the many-to-one traffic pattern where many nodes send data towards a border router or sink node, but provides a set of mechanisms for any-to-any routing as well.
RPL supports three type of communication: MP2P (Multipoint-to-Point), P2P (Point-to-Point) and P2MP (Point-to-Multipoint). MP2P flow of traffic is common in LLNs, and each node of the graph has reachability toward the graph root; the MP2P flow of traffic is called the ’up’ direction in the DODAG. Similar to MP2P, P2MP is called the ’down’ direction.
This traffic may originate from outside the LLN network, at the root or at any intermediate nodes and destined to a (leaf) node. Finally the P2P communication is from any node to any other node in the graph. When a node sends a packet to another node within the LLN, the packet travels ’up’ to a common ancestor at which point it is forwarded in the ’down’ direction to the destination.
A Directed Acyclic Graph (DAG) is an on oriented graph with no directed cycles, and is formed by a collection of vertices and directed edges. Each edge connects one vertex to another, so there is no way to start at some vertex v and follow a sequence of edges that eventually loops back to v again. A DAG root is a node within the DAG that has no outgoing edge; because the graph is acyclic, by definition all DAGs must have at least one DAG root and all paths terminate at a DAG root. A DAG with an unique root node forms a Destination Oriented DAG (DODAG). RPL protocol creates a DODAG structure looks like a tree where the root is named DODAG root; in this mode each node could became parent, child, ancestor or leaf. A network can include different DODAGs, and RPL uses four values to identify and maintain a topology:
1 RPLInstanceID: A RPLInstanceID identifies a set of one or more DODAGs. A network may have multiple RPLInstanceIDs, each of which defines an independent set of DODAGs, which may be optimized for different Objective Functions (OFs) and/or applications. The set of DODAGs identified by a RPLInstanceID is called a RPL Instance. All DODAGs in the same RPL Instance use the same OF.
2 DODAGID: Identify the DODAG root and the corresponding DODAG. The combination of RPLInstanceID and DODAGID uniquely identifies a single DODAG in the network.
3 DODAGVersionNumber: A DODAG is sometimes reconstructed from the DODAG root, by incrementing the DODAGVersionNumber. The combination of RPLInstanceID, DODAGID, and DODAGVersionNumber uniquely identifies a DODAG Version.
4 RANK: Rank defining individual node positions with respect to the DODAG root, and strictly increases in the ’down’ direction and strictly decreases in the ’up’ direction.
RPL also provides the ability to perform multi-topology routing (MTR) thanks to the concept of a DODAG instance identified by an instance-id. The idea is to construct and identify multiple graphs (DODAGs) over the same physical topology. This provides a way to built paths based on different optimization objectives as specified by the Objective Function and the routing/constraint metrics. A node can only join a single graph within an instance-id but can be associated with several instance-ids simultaneously.