8.5. Operation as a Leaf Node

In some cases, a RPL node may attach to a DODAG as a leaf node only. One example of such a case is when a node does not understand or does not support (policy) the RPL Instance's OF or advertised metric/ constraint. As specified in Section 18.6, related to policy function, the node may either join the DODAG as a leaf node or may not join the DODAG. As mentioned in Section 18.5, it is then recommended to log a fault.

A leaf node does not extend DODAG connectivity; however, in some cases, the leaf node may still need to transmit DIOs on occasion, in particular, when the leaf node may not have always been acting as a leaf node and an inconsistency is detected.

A node operating as a leaf node must obey the following rules:

1. It MUST NOT transmit DIOs containing the DAG Metric Container.

2. Its DIOs MUST advertise a DAGRank of INFINITE_RANK.

3. It MAY suppress DIO transmission, unless the DIO transmission has been triggered due to detection of inconsistency when a packet is being forwarded or in response to a unicast DIS message, in which case the DIO transmission MUST NOT be suppressed.

4. It MAY transmit unicast DAOs as described in Section 9.2.

5. It MAY transmit multicast DAOs to the '1 hop' neighborhood as described in Section 9.10.

A particular case that requires a leaf node to send a DIO is if that leaf node was a prior member of another DODAG and another node forwards a message assuming the old topology, triggering an inconsistency. The leaf node needs to transmit a DIO in order to repair the inconsistency. Note that due to the lossy nature of LLNs, even though the leaf node may have optimistically poisoned its routes by advertising a Rank of INFINITE_RANK in the old DODAG prior to becoming a leaf node, that advertisement may have become lost and a leaf node must be capable to send a DIO later in order to repair the inconsistency.

In the general case, the leaf node MUST NOT advertise itself as a router (i.e., send DIOs).