short paragraph and diagram on the NTP network

selected/time.md

@@ -140,7 +140,16 @@ With all these concepts established, the question now is how to actually represe
 # What time is it actually: time synchronization
     - NTP/chrony
     - forkingpaths help you are the one who is the smart
+TODO: short intro
 
+## The NTP Network
+To maintain a consistent notion of time accross the internet, you first need some reference time signal, which is typically provided by atomic clocks. Since it is neiter practical nor desirable to connect every computer in the world to these atomic clocks, the time signal is distributed accross a network of peers organized in serveral strata. Primary servers sit at stratum 1 and are directly connected (through wire or radio) to a reference clock. Peers at strata 2 can synchronize on servers at strata 1 or 2, and act as a synchronization sources for peers at strata 3. The layering continues up to stratum 15. Strata 16 represents unsynchronized devices.
+   
+![NTP architecture flow graph](./NTP_hierarchy.svg)
+
+The protocol selects sources in order to avoid loops and minimize the round-trip delay to a primary server. As such peers collaborate to automatically reorganize the network to produce the most accurate time, even in the presence of failures in the network.  
+
+## Peer Architecture
 ![NTP architecture flow graph](./NTP_architecture_summary.svg)
 
 # How long things take: durations