13. Maintenance of Routing Adjacency

The selection of successors, along the default paths Up along the DODAG, or along the paths learned from destination advertisements Down along the DODAG, leads to the formation of routing adjacencies that require maintenance.

In IGPs, such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of a routing adjacency involves the use of keepalive mechanisms (Hellos) or other protocols such as the Bidirectional Forwarding Detection (BFD) [RFC5881] and the MANET Neighborhood Discovery Protocol (NHDP) [RFC6130]. Unfortunately, such a proactive approach is often not desirable in constrained environments where it would lead to excessive control traffic in light of the data traffic with a negative impact on both link loads and nodes resources.

By contrast with those routing protocols, RPL does not define any keepalive mechanisms to detect routing adjacency failures: this is because in many cases, such a mechanism would be too expensive in terms of bandwidth and, even more importantly, energy (a battery-operated device could not afford to send periodic keepalives). Still RPL requires an external mechanisms to detect that a neighbor is no longer reachable. Such a mechanism should preferably be reactive to traffic in order to minimize the overhead to maintain the routing adjacency and focus on links that are actually being used.

Example reactive mechanisms that can be used include:

• The Neighbor Unreachability Detection [RFC4861] mechanism.

• Layer 2 triggers [RFC5184] derived from events such as association states and L2 acknowledgements.