The ETX of a link is the predicted number of data transmissions required to send a packet over that link, including retransmissions. The ETX of a route is the sum of the ETX for each link in the route. For example, the ETX of a three-hop route with perfect links is three; the ETX of a one-hop route with a 50% delivery ratio is two.
The ETX of a link is calculated using the forward and reverse delivery ratios of the link. The forward delivery ratio, df , is the measured probability that a data packet successfully arrives at the recipient; the reverse delivery ratio, dr , is the probability that the ACK packet is successfully received. The expected probability that a transmission is successfully received and acknowledged is df x dr .
A sender will retransmit a packet that is not successfully acknowledged. Because each attempt to transmit a packet can be considered a Bernoulli trial, the expected number of transmissions is:
ETX = 1/ (df x dr)
For a full description of how df and dr is actually calculated see MIT's publication site