ROLL WG Omprakash Gnawali and Philip Levis Stanford University 6/16/2010

# ROLL WG Omprakash Gnawali and Philip Levis Stanford University 6/16/2010

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## ROLL WG Omprakash Gnawali and Philip Levis Stanford University 6/16/2010

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1. The Minimum Rank Objective Function with Hysteresis(MRHOF)draft-gnawali-roll-minrank-hysteresis-of-00 ROLL WG Omprakash Gnawali and Philip Levis Stanford University 6/16/2010

2. RPL and Objective Functions • Objective functions • Compute the rank based on metrics • Select parent among candidate next hops • Metric Examples • Hop count (OF0) • Throughput • ETX • De-coupled from the metrics

3. The Minimum Rank Objective Function with Hysteresis (MRHOF) • Applicable metrics • Listed in I-D.ietf-roll-routing-metrics • Additive • Global • Metric minimization as routing objective • Based on experiences with ETX • Might be applicable to latency and other metrics

4. Some Terminology • Selected metric • The routing metric to minimize • Path metric • Composed of link metrics over the links on a path • Is path metric the same as rank? • How to translate ETX (NTX) or latency (ms) to rank?

5. Metric Minimization • Compute the Path metric and parent • At the root • cur_min_path_metric = MIN_PATH_METRIC • On other nodes • candidate_min_path_metric(i) =Link metric to a candidate i + Path metric from ito the DAG root • cur_min_path_metric = Min{candidate_min_path_metric(1..n)} • Parent = candidate whose cost is equal to cur_min_path_metric • Examples • ETX: Paths with the smallest expected transmit count • Latency: Least latency paths

6. Hysteresis • Link metric can have jitter • Example: ETX jitter due to changing link quality • Can cause churn in the topology • Hysteresis delays the effects • Short-term and small changes in link properties should not trigger path recomputation • Change parent if the new path metric is better by at least PARENT_SWITCH_THRESHOLD

7. Other Considerations • Frequency of updates • Periodic • Reactive • Metric-specific parameters • Max and min • Route switching threshold • Tie-breaker