The ETX Objective Function
draft-gnawali-roll-etxof-00
ROLL WG
IETF 77 - Anaheim, CA
3/22/2010
Omprakash Gnawali and Philip Levis
Stanford University
RPL and Objective Functions
• Objective functions are used to select next
hop among candidate next hops
• Examples
– Hop count (OF0)
– Throughput
– ETX
• Combinations possible
The ETX Metric
• ETX = Expected Number of Transmissions
– Includes ReTX and ACK
• Path ETX = Sum of Link ETX
• Link ETX described in draft-ietf-rollrouting-metrics
• Many ways to compute ETX
• Widely used in wireless routing
– Several MANET protocols, CTP
The ETX Objective Function (ETXOF)
• Pick the candidate with the smallest Path ETX
– Path ETX = 0 at DAG root
– Path ETX = Link ETX to a candidate n + Path ETX
from n to the DAG root
• Some considerations
– Frequency of updates
– Hysteresis to prevent excessive churn
– Path and link ETX thresholds
– Tie-breaker
RPL and ETXOF
• ETXOF applicable to many RPL deployments
• Adopt draft-gnawali-roll-etxof as
a WG document?