8. Upward Routes

This section describes how RPL discovers and maintains Upward routes. It describes the use of DODAG Information Objects (DIOs), the messages used to discover and maintain these routes. It specifies how RPL generates and responds to DIOs. It also describes DODAG Information Solicitation (DIS) messages, which are used to trigger DIO transmissions.

As mentioned in Section 3.2.8, nodes that decide to join a DODAG MUST provision at least one DODAG parent as a default route for the associated instance. This default route enables a packet to be forwarded Upward until it eventually hits a common ancestor from which it will be routed Downward to the destination. If the destination is not in the DODAG, then the DODAG root may be able to forward the packet using connectivity to the outside of the DODAG; if it cannot forward the packet outside, then the DODAG root has to drop it.

A DIO message can also transport explicit routing information:

DODAGID: The DODAGID is a Global or Unique Local IPv6 address of the root. A node that joins a DODAG SHOULD provision a host route via a DODAG parent to the address used by the root as the DODAGID.

RIO Prefix: The root MAY place one or more Route Information options in a DIO message. The RIO is used to advertise an external route that is reachable via the root, associated with a preference, as presented in Section 6.7.5, which incorporates the RIO from [RFC4191]. It is interpreted as a capability of the root as opposed to a routing advertisement, and it MUST NOT be redistributed in another routing protocol though it SHOULD be used by an ingress RPL router to select a DODAG when a packet is injected in a RPL domain from a node attached to that RPL router. An Objective Function MAY use the routes advertised in RIO or the preference for those routes in order to favor a DODAG versus another one for the same instance.