3.3. Downward Routes and Destination Advertisement

RPL uses Destination Advertisement Object (DAO) messages to establish Downward routes. DAO messages are an optional feature for applications that require point-to-multipoint (P2MP) or point-to-point (P2P) traffic. RPL supports two modes of Downward traffic: Storing (fully stateful) or Non-Storing (fully source routed); see Section 9. Any given RPL Instance is either storing or non-storing. In both cases, P2P packets travel Up toward a DODAG root then Down to the final destination (unless the destination is on the Upward route). In the Non-Storing case, the packet will travel all the way to a DODAG root before traveling Down. In the Storing case, the packet may be directed Down towards the destination by a common ancestor of the source and the destination prior to reaching a DODAG root.

As of the writing of this specification, no implementation is expected to support both Storing and Non-Storing modes of operation. Most implementations are expected to support either no Downward routes, Non-Storing mode only, or Storing mode only. Other modes of operation, such as a hybrid mix of Storing and Non-Storing mode, are out of scope for this specification and may be described in other companion specifications.

This specification describes a basic mode of operation in support of P2P traffic. Note that more optimized P2P solutions may be described in companion specifications.