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Network Time Protocol - NTP

Network Time Protocol - NTP. Lizandro Damian Solano-Quinde. NTP - Introduction. NTP is a protocol for synchronizing the clocks on computers over packet-switched data networks NTP delivers accurate and reliable time in spite of faults in the network

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Network Time Protocol - NTP

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  1. Network Time Protocol - NTP Lizandro Damian Solano-Quinde

  2. NTP - Introduction • NTP is a protocol for synchronizing the clocks on computers over packet-switched data networks • NTP delivers accurate and reliable time in spite of faults in the network • Provides a connectionless service (UDP in the Transport Layer) • NTP is used on Internet

  3. NTP - Clock Strata • NTP uses a hierarchical organization of clocks • Stratum 0.- Composed by: Atomic Clocks, GPS Clocks. • Stratum 1 - Primary • Time Servers.- Computers attached to stratus 0 devices • They act as servers for requests from Stratus 2 • Stratum 2 • Computers sending NTP requests to Time Servers in Stratum 1 • Computers in this level will reference to several time servers to synchronize their clocks • S2 Computers will peer with another S2 computers to provide more reliable and robust time for all devices in the peer group • They act as servers for requests from Stratus 3

  4. NTP - Clock Strata • Stratum 3, 4, … • Computers employ the same NTP function as in Stratum 2 • Potentially up to 16 levels * Image taken from www.wikipedia.com

  5. NTP - Synchronization Subnet • Computer local clock are synchronized to a number of Time Servers and peer computer • The set of these computers and Time Servers is known as the Synchronization Subnet • The Stratum Number for each computer is determined by the hop count to the root (Strata 0)

  6. NTP - Synchronization Subnet • Example:

  7. NTP - Determining Time • Timestamps exchanged between the server and clients (subnet peers)

  8. NTP - Determining Time • Examples:

  9. NTP - Determining Time • Bounds for Clock Offset

  10. NTP - Peer Selection & Combining Algorithms • Definitions: • Synchronization Distance: Roundtrip path delay to the root • Synchronization Dispersion: Total dispersion to the root. Dispersion is defined by:

  11. NTP - Peer Selection & Combining Algorithms • Assumptions: • The best offset samples should occur at the lowest delays • The highest reliability is usually associated with the lowest stratum and synchronization dispersion.

  12. NTP - Peer Selection & Combining Algorithms • Filtering • The candidate list is sorted by stratum and synchronization dispersion • A sanity check is performed • If no peers pass the sanity check, the algorithm is aborted and no updates are made • List is pruned to obtain one with a maximum predetermined size (m)

  13. NTP - Peer Selection & Combining Algorithms • Faulty clocks detection • The candidate list is sorted by stratum and synchronization distance • For each candidate j (0 ≤ j ≤ m) compute the Select Dispersion: • Discard the candidate with the maximum єj and repeat this faulty clocks detection algorithm • The algorithm stops when all the maximum selection dispersion over all the remaining candidates is less than the minimum value or there is only one candidate remaining in the list

  14. NTP - Peer Selection & Combining Algorithms

  15. NTP - Modes of Operation • Multicast • For high speed networks • High Accuracies are not required • Time Servers send periodic NTP broadcasts • Determine the time based on an assumed delay • Time servers provides synchronization, but do not accept NTP messages from clients

  16. NTP - Modes of Operation • Procedure-Call • Intended for file servers or workstations that require high levels of accuracy • A Time Server acting as a client send a request to a peer operating as a server • Server after inserting timestamps and recalculating the checksum, sends back the message • A server operating as a client can be synchronized, but cannot provide synchronization • A server operating as a server can provide synchronization, but cannot be synchronized

  17. NTP - Modes of Operation • Symmetric • A server can provide synchronization or to be synchronized • Two modes of operation: • Active Mode: For servers in the high levels of the stratum (near the leaves) • Passive Mode: For servers in the low levels of the stratum (near the root)

  18. NTP - References • [ 1 ] D. Mills, “Internet Time Synchronization: The Network Time Protocol”, IEEE Trans. on Communications, vol. 39, no. 10, pp. 1482 – 1493, Oct. 1991. • [ 2 ] NTP: The Network Time Protocol - www.ntp.org • [ 3 ] Fault Tolerant Time Synchronization - www.eecis.udel.edu/~millslab • [ 4 ] Wikipedia - www.wikipedia.com

  19. NTP - References Questions ?

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