How ETX (Estimated Transmissions) is implemented in contikiRPL? - moving-average

Question

I am working on creating a modified version of MRHOF for RPL. However, I
have some doubts about the ETX metrics used. i am running an rpl-udp example (..../contiki-3.0/examples/ipv6/rpl-udp).
As per my understanding, the general definition of ETX is following:
ETX = 1/(df * dr)
where df is the measured probability that a data packet successfully arrives at the recipient and dr is the probability that the ACK packet is successfully received.
The implementation of ETX is defined in neighbor_link_callback(rpl_parent_t *p, int status, int numtx) (contiki/core/net/rpl/rpl-mrhof.c) as below:
new_etx = ((uint32_t)recorded_etx * ETX_ALPHA +(uint32_t)packet_etx * (ETX_SCALE - ETX_ALPHA)) / ETX_SCALE
where
recorded_etx = nbr->link_metric
packet_etx = MAX_LINK_METRIC * RPL_DAG_MC_ETX_DIVISOR
nbr->link_metric = RPL_INIT_LINK_METRIC * RPL_DAG_MC_ETX_DIVISOR (rpl-dag.c)
RPL_INIT_LINK_METRIC = 2 (rpl-conf.h)
ETX_SCALE = 100
ETX_ALPHA = 90
RPL_DAG_MC_ETX_DIVISOR = 256 (rpl-private.h)
MAX_LINK_METRIC = 10
Here every time when link layer receives an ACK or time-out event the function inside this file (neighbor_link_callback) is fired.
I understood the formal definition of ETX, but when i am trying to map the standard ETX formula with contikiRPL's ETX formula then i am facing some trouble in understanding the implementation of ETX in contikiRPL.
How the probability of a data packet successfully arrives at the recipient (df) and probability that the ACK packet is successfully received (dr) are implemented in ContikiRPL?

Answer 1

In the code, df and dr individually are not known. The algorithm is run on the sender device, which has no means to differentiate between the case when the packet is lost and the case when the ACK is lost. They look exactly the same to it: as the absence of the
The value of packet_etx roughly corresponds to 1 / (df * dr) of the last packet. Note that a single packet already may have had multiple retransmissions on the link. The metric is updated only when the packet is successfully ACKed or when the maximal number of retransmissions is exceeded.
Another issue in Contiki is that since its designed for embedded systems, it does not have the memory to keep in track the ETX of many recent packets. Instead, this information is aggregated in single value with the help of exponentially weighted moving average (EWMA) filter. The \alpha paramter of the algorithm is given as ETX_ALPHA / ETX_SCALE in the code; the scaling is done to avoid the more expensive floating point operations.
The value of recorded_etx is the previous value of the ETX, reflecting the ETX calculated from all of the previous packets. The value of new_etx is the value of the link's ETX when the previous ETX and the last packet's ETX have been combined with the ETX algorithm.