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RPL is designed for Low Power and Lossy Wireless Networks such as Wireless Sensor Networks. It hence it optimized for collection networks (where nodes periodically report measurements to a small number of collection points) with infrequent communication from the collection point to individual nodes. RPL dubs collection traffic Multi-Point-to-Point (MP2P) and configuration traffic Point-To-Multi-Point (P2MP). RPL does not support Point-To-Point (P2P) traffic well, although workarounds can be used.
For MP2P traffic, RPL builds a gradient into the network, grounded at the collection points called Low power and lossy network Border Router (LBR). Each node is assigned a rank such that rank increases as the node is far from the LBR. In RPL, this gradient is called a Destination Oriented Directed Acyclic Graph (DODAG). Forwarding a packet to the LBR roughly consist in picking the neighbor node with the lowest rank.
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