The Minimum Rank Objective Function with Hysteresis

An objective function allows RPL to optimize or constrain the routing metric of a path. RPL achieves this goal by selecting the parent among the alternate parents as dictated by that objective function. For example, if an RPL instance uses an objective function that minimizes hop-count, RPL will select paths with minimum hop count. Different objective functions optimize or constrain metrics differently. The nodes running RPL might use a number of metrics to describe a link and make it available for route selection. A metric can be used by different objective functions to optimize or constrain the metric in different ways. This specification describes MRHOF, an objective function for RPL. MRHOF uses hysteresis while selecting the path with the smallest metric value. The path with the minimum metric value has different property depending on the metric used for path selection. For example, the use of MRHOF with the latency metric allows RPL to find stable minimum-latency paths from the nodes to a root in the DAG instance. The use of MRHOF with the ETX metric allows RPL to find the stable minimum-ETX paths from the nodes to a root in the DAG instance. MRHOF can be used with additive metric that must be minimized on the paths selected for routing. Although MRHOF can be used with a number of metrics, this draft is based on experiences with the ETX metric.

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. This terminology used in this document is consistent with the terminologies described in , , and . This document introduces two term: The metric chosen by the network operator to use for path selection. This metric can be any additive metric listed in Path metric quantifies a property of an end-to-end path. Path metric is composed using the selected metric of the links along the path. Path metrics can be used by RPL to compare different paths.

The Minimum Rank Objective Function with Hysteresis, MRHOF, is designed to find the paths with the smallest metric values while preventing excessive churn in the network. It does so by switching to the minimum metric path only if the path metric of the current path is larger than the path metric of the minimum metric path by a given threshold. MRHOF may be used with any additive metric listed in as long the routing objective is to minimize the given routing metric. MRHOF cannot be used if the routing objective is to maximize the metric.

Nodes compute the path metric for each candidate neighbor reachable on all the interfaces. The Path metric represents the cost of the path, in terms of the selected metric, from a node to the root of the DODAG through the neighbor. Root nodes (Grounded or Floating) set the variable cur_min_path_metric to MIN_PATH_METRIC. A non-root node computes the path metric for a path to the root through each candidate neighbor by adding these two components: The selected metric for the link to a candidate neighbor. The cur_min_path_metric advertised by that neighbor. A node SHOULD compute the path metric for the path through each candidate neighbor reachable through all interfaces. If a node cannot compute the path metric for the path through a candidate neighbor, the node MUST NOT make that candidate neighbor its preferred parent. If the selected metric of the link to a neighbor is not available, the path metric for the path through that neighbor SHOULD be set to MAX_PATH_METRIC. This metric value will prevent this path from being considered for path selection. The path metric corresponding to a neighbor MUST be re-computed each time: The selected metric of the link to the candidate neighbor is updated. A node receives a new cur_min_path_metric advertisement from the candidate neighbor. This computation MAY also be performed periodically. However, long intervals between periodic computation or deferring the computation for too long after new cur_min_path_metric advertisements or updates to the selected link metric prevents results in node making parent selection based on stale link and path information.

After computing the path metric for all the candidate neighbors reachable through all the interfaces for the current DODAG iteration, a node selects the preferred parent. This process is called parent selection. A node MUST select a candidate neighbor as its preferred parent if the path metric corresponding to that neighbor is smaller than the path metric corresponding to the rest of the neighbors, except as indicated below: If the smallest path metric for paths through the candidate neighbors is smaller than cur_min_path_metric by less than PARENT_SWITCH_THRESHOLD, the node MAY continue to use the current preferred parent. If there are multiple paths with the smallest path metric and that smallest path metric is smaller than cur_min_path_metric by at least PARENT_SWITCH_THRESHOLD, a node MAY use a different objective function to select the preferred parent among the candidates which are first hop on the path with the smallest path metric. A node MAY declare itself as a Floating root, and hence no preferred parent, depending on the configuration. If the selected metric for a link is greater than MAX_LINK_METRIC, the node SHOULD exclude that link from consideration for parent selection. If cur_min_path_metric is greater than MAX_PATH_METRIC, the node MAY declare itself as a Floating root. If the configuration disallows a node to be a Floating root and no neighbors are discovered, the node does not have a preferred parent, and MUST set cur_min_path_metric to MAX_PATH_METRIC.

Once the preferred parent is selected, the node sets its cur_min_path_metric variable to the path metric corresponding to the preferred parent. Thus, cur_min_path_metric is the cost of the path with the smallest path metric from the node to the root. The value of the cur_min_path_metric is carried in the metric container whenever DIO messages are sent.

MRHOF uses the following variable: cur_min_path_metric: The path metric of the path from a node through its preferred parent to the root computed at the last parent selection. MRHOF uses the following parameters: MAX_LINK_METRIC: Maximum allowed value for the selected link metric for each link on the path. MAX_PATH_METRIC: Maximum allowed value for the path metric of a selected path. MIN_PATH_METRIC: The minimum allowed value for the path metric of the selected path. PARENT_SWITCH_THRESHOLD: The difference between metric of the path through the preferred parent and the minimum-metric path to trigger new preferred parent selection. The parameter values are assigned depending on the selected metric. Here is an example parameter assignment for the ETX metric: MAX_LINK_METRIC: 10. Disallow links with greater than 10 expected transmission count on the selected path. MAX_PATH_METRIC: 100. Disallow paths with greater than 100 expected transmission count. MIN_PATH_METRIC: 0. At root, the expected transmission count is 0. PARENT_SWITCH_THRESHOLD: 1.0. Switch to a new path only if it is requires at least one fewer transmission than the current path.

This specification requires an allocated OCP. A value of 1 is requested.

Security considerations to be developed in accordance to the output of the WG.