2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
Additionally, this document uses terminology from [ROLL-TERMS], and introduces the following terminology:
DAG: : Directed Acyclic Graph. A directed graph having the property that all edges are oriented in such a way that no cycles exist. All edges are contained in paths oriented toward and terminating at one or more root nodes.
DAG root: : A DAG root is a node within the DAG that has no outgoing edge. Because the graph is acyclic, by definition, all DAGs must have at least one DAG root and all paths terminate at a DAG root.
Destination-Oriented DAG (DODAG): : A DAG rooted at a single destination, i.e., at a single DAG root (the DODAG root) with no outgoing edges.
DODAG root: : A DODAG root is the DAG root of a DODAG. The DODAG root may act as a border router for the DODAG; in particular, it may aggregate routes in the DODAG and may redistribute DODAG routes into other routing protocols.
Virtual DODAG root: : A Virtual DODAG root is the result of two or more RPL routers, for instance, 6LoWPAN Border Routers (6LBRs), coordinating to synchronize DODAG state and act in concert as if they are a single DODAG root (with multiple interfaces), with respect to the LLN. The coordination most likely occurs between powered devices over a reliable transit link, and the details of that scheme are out of scope for this specification (to be defined in future companion specifications).
Up: : Up refers to the direction from leaf nodes towards DODAG roots, following DODAG edges. This follows the common terminology used in graphs and depth-first-search, where vertices further from the root are "deeper" or "down" and vertices closer to the root are "shallower" or "up".
Down: : Down refers to the direction from DODAG roots towards leaf nodes, in the reverse direction of DODAG edges. This follows the common terminology used in graphs and depth-first-search, where vertices further from the root are "deeper" or "down" and vertices closer to the root are "shallower" or "up".
Rank: : A node's Rank defines the node's individual position relative to other nodes with respect to a DODAG root. Rank strictly increases in the Down direction and strictly decreases in the Up direction. The exact way Rank is computed depends on the DAG's Objective Function (OF). The Rank may analogously track a simple topological distance, may be calculated as a function of link metrics, and may consider other properties such as constraints.
Objective Function (OF): : An OF defines how routing metrics, optimization objectives, and related functions are used to compute Rank. Furthermore, the OF dictates how parents in the DODAG are selected and, thus, the DODAG formation.
Objective Code Point (OCP): : An OCP is an identifier that indicates which Objective Function the DODAG uses.
RPLInstanceID: : A RPLInstanceID is a unique identifier within a network. DODAGs with the same RPLInstanceID share the same Objective Function.
RPL Instance: : A RPL Instance is a set of one or more DODAGs that share a RPLInstanceID. At most, a RPL node can belong to one DODAG in a RPL Instance. Each RPL Instance operates independently of other RPL Instances. This document describes operation within a single RPL Instance.
DODAGID: : A DODAGID is the identifier of a DODAG root. The DODAGID is unique within the scope of a RPL Instance in the LLN. The tuple (RPLInstanceID, DODAGID) uniquely identifies a DODAG.
DODAG Version: : A DODAG Version is a specific iteration ("Version") of a DODAG with a given DODAGID.
DODAGVersionNumber: : A DODAGVersionNumber is a sequential counter that is incremented by the root to form a new Version of a DODAG. A DODAG Version is identified uniquely by the (RPLInstanceID, DODAGID, DODAGVersionNumber) tuple.
Goal: : The Goal is an application-specific goal that is defined outside the scope of RPL. Any node that roots a DODAG will need to know about this Goal to decide whether or not the Goal can be satisfied. A typical Goal is to construct the DODAG according to a specific Objective Function and to keep connectivity to a set of hosts (e.g., to use an Objective Function that minimizes a metric and is connected to a specific database host to store the collected data).
Grounded: : A DODAG is grounded when the DODAG root can satisfy the Goal.
Floating: : A DODAG is floating if it is not grounded. A floating DODAG is not expected to have the properties required to satisfy the goal. It may, however, provide connectivity to other nodes within the DODAG.
DODAG parent: : A parent of a node within a DODAG is one of the immediate successors of the node on a path towards the DODAG root. A DODAG parent's Rank is lower than the node's. (See Section 3.5.1).
Sub-DODAG: : The sub-DODAG of a node is the set of other nodes whose paths to the DODAG root pass through that node. Nodes in the sub-DODAG of a node have a greater Rank than that node. (See Section 3.5.1).
Local DODAG: : Local DODAGs contain one and only one root node, and they allow that single root node to allocate and manage a RPL Instance, identified by a local RPLInstanceID, without coordination with other nodes. Typically, this is done in order to optimize routes to a destination within the LLN. (See Section 5).
Global DODAG: : A Global DODAG uses a global RPLInstanceID that may be coordinated among several other nodes. (See Section 5).
DIO: : DODAG Information Object (see Section 6.3)
DAO: : Destination Advertisement Object (see Section 6.4)
DIS: : DODAG Information Solicitation (see Section 6.2)
CC: : Consistency Check (see Section 6.6)
As they form networks, LLN devices often mix the roles of host and router when compared to traditional IP networks. In this document, "host" refers to an LLN device that can generate but does not forward RPL traffic; "router" refers to an LLN device that can forward as well as generate RPL traffic; and "node" refers to any RPL device, either a host or a router.