Lesson plan
Intersection algorithm
The intersection algorithm is an agreement algorithm used to select sources for estimating accurate time from a number of noisy time sources. It forms part of the modern Network Time Protocol. It is a modified form of Marzullo's algorithm. While Marzullo's algorithm will return the smallest interval consistent with the largest number of sources, the returned interval does not necessarily include the center point (calculated offset) of all the sources in the intersection. The intersection algorithm returns an interval that includes that returned by Marzullo's algorithm but may be larger since it will include the center points. This larger interval allows using additional statistical data to select a point within the interval, reducing the jitter in repeated execution.
Article summary
The intersection algorithm is an agreement algorithm used to select sources for estimating accurate time from a number of noisy time sources. It forms part of the modern Network Time Protocol. It is a modified form of Marzullo's algorithm. While Marzullo's algorithm will return the smallest interval consistent with the largest number of sources, the returned interval does not necessarily include the center point (calculated offset) of all the sources in the intersection. The intersection algorithm returns an interval that includes that returned by Marzullo's algorithm but may be larger since it will include the center points. This larger interval allows using additional statistical data to select a point within the interval, reducing the jitter in repeated execution.
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
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