Re: [Roll] ETX metric

[Philip Levis <pal at cs.stanford.edu>]

I think we should keep in mind that RPL must work across multiple link  
layers. E.g.:

A -- 900MHz --> B -- 2.4GHz --> C

ETX is a commonly used metric, but typically under two simplifying  
assumptions:

1) Routes are composed of a single link layer
2) The link layer has a static bitrate

If you break either assumption, it doesn't work well. ETT (expected  
time of transmission) can address 2), but not 1).

I'm wary of encoding a metric as a hard quantity. For example,  
quantifying the metric as uJ breaks when nodes have differential  
energy capacities. What we really want is a more abstract notion of  
cost, which a particular device can use to, in a very simple way,  
express its own tradeoffs. It is critical that exactly how devices  
calculate this value remain unspecified.

My first thought would be a quantification of how much of a node's  
"lifetime" a packet would cost. Such a cost can consider both the  
receiver and transmitter. E.g., given its particular low power  
algorithms and expected lifetime, how much would sending to this  
destination consume? (I try to avoid using the word "link.")

I don't think that a single metric will be sufficient; besides some  
notion of cost (hopcount, ETX, ETT, etc.), the other metric that is  
critical in many applications is latency. These two -- cost and  
latency -- can cover a large portion of the application design space,  
and provide a sound basis for the basic specification.

Thoughts?

Phil