report_net_timing results values and units?

[Altera_Forum]

I've got various ring oscillator circuits that I'd like to get delay timing information for. 

report_net_timing on the delays in a ring gives results in a table like the one below:   ;# ; Node Name ; RR ; FR ; RF ; FF ; Destination Pin Name ; Location ; ; 1 ; "myNode2"|combout ; 0.207 ; ---- ; ---- ; 0.185 ; "myNode3"|datac ; LABCELL_X27_Y12_N36 ; ; 1 ; "myNode1"|combout ; 0.212 ; ---- ; ---- ; 0.178 ; "myNode2"|datac ; MLABCELL_X28_Y12_N36     I can't seem to find what RR, FR, RF, and FF stand for or what are the units for those values (RR,FR,RF,FF) in the table.  Any help or direction would be appreciated!  (FYI, using Arria 10)

[Altera_Forum]

RR=Rise -> Rise, when a rising edge into the LUT creates a rising edge out. Basically rise/fall all have different delays, so it needs to account for all of this.

[Altera_Forum]

To my understanding, TimeQuest also does unateness analysis, which is looking at what's possible. For example, an AND gate will only have RR and FF, as a rising edge cannot create a falling output, and vice versa. Something like an XOR gate can do them all.

[Altera_Forum]

Thank you! Any ideas on the exact units for the delays given?

[Altera_Forum]

nanoseconds. Note that there is large variation in delays, so your ring oscillator could have very large frequency differences over PVT(which is why ring oscillators are not recommended except for corner cases where you're doing some non-synchronous circuit). So if asked about ring oscillators, I lead off by saying, "Don't do it", but if you really want to, I put together the following document: 

http://www.alterawiki.com/wiki/ring_oscillator