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NTP (Clock strata (Each level of the hierarchy is a stratum (Stratum 3…

- - - - Syncrhonized to stratum 2 servers
      - Employ same algorithms for peering and data sampling as stratum 2
      - Can themselves act as servers for stratum 4 computers
    - - Computer that are synchronized over a network to stratum 1 servers
      - Often a stratum 2 computer will query several stratum 1 servers
      - May peer with other stratum 2 computers to provide more stable and robust time
    - - Computers whose system tume is synchronized to within a few microseconds
      - Referred to as primary time servers
      - Stratum 1 servers may peer with other stratum 1 servers for sanity check and backup
    - - High-precission timekeeping devices
        
        Atomic clocks
        
        GPS
        
        Radio clocks
      - Also known as reference clock
      - Generate very accurate pulse per second signal that triggers an interrupt and timestamp
- - - - Described client-server & peer-to-peer algorithms
      - Much more complete specification of the NTPv1
    - - NTPv2
      - Published by means of a state machine
      - With pseudocode to describe its operation
    - - NTPv3
      - Included an analysis of all sources of error
      - Enabled a calculation of a metric that helps choose the best server where several candidates appear to disagree
    - - NTPv4
- - - - Where both peers consider the other to be a potential time source