Network Time Protocol

• NTP is a protocol for clock synchronization over packet switched, variable latency data networks
• syncs within miliseconds of each other using UTC
• uses Marzullo's algorithm, to estimate correct time from noisy sources
  • produces an optimal value from a set of estimates with confidence intervals
  • the best estimate is taken to be the smallest interval consistent with the largest number of sources
• NTP can maintain time within tens of miliseconds, passing timestamps through UDP

Hybrid Logical Clock and Consistent Snapshots

• aims to provide tighter synchronization than using physical/NTP clocks
• enables consistent snapshots of distributed systems
• history; logical clocks pass information instead of physical time
  • impractical because cannot query based on physical time, and is not tolerent
• vector clocks were proposed to maintain a vector at each node which tracks the logical clocks of other nodes
  • while LC finds one consistent snapshot from nodes, VC finds all possible snapshots
• Physical Time uses physical clocks, with uncertainty intervals
  • overlapping intervals cannot determine order
  • poor network conditions can result in async