[net2-wg] link estimator interface

[Omprakash Gnawali]

I was looking at the Moteiv SP interfaces, and I wanted to discuss the
notion of link quality provided by these interfaces:

getQuality 
command uint16_t getQuality(sp_neighbor_t *n, TOS_Msg *msg) 

The documentation says that this call is supposed to return ETX. That
is one way to standardize the return value so that any routing
protocol can use this information without having to adapt to different
link quality estimators.

What if someone just wants to use LQI based estimator. In that case,
not only is the metric asymmetric, but it would be impossible to
provide the above command because many people seem to have come up
with very different correlation between ETX/reliability and LQI
depending on the environment. Even in a single environment, I am not
sure if there is a strong correlation between these variables.

What do people think about abstracting the notion of link quality and
saying here is the range of values and the values in between describe
link quality in linear proportion of the range? For example,
1/2(max+min) would describe link quality roughly in between the
maximum and minimum quality.

Without standardizing the return value, I do not see how different
routing protocols can work with a variety of link estimators. The
drawback is this will make it harder for protocols that rely on a
specific implementation of an estimator - protocols that configure the
number of retransmission based on ETX, for example.