Outline for June 4, 1997

  1. Greetings and Felicitations
  2. Example Analysis: NTP v 2
    1. Protocol to synchronize clocks on the Internet
    2. Primary time servers: synced to an external time system (e.g., atomic clocks)
    3. Go through cloud hierarchy (and cohorts), stratum layer (distance from primary) , synchronization
    4. Association modes: client (sends message to peer, can reset time), server (replies to sent message), broadcast (send out periodic messages); higher up, symmetric active and symmetric passive: association periodically sends out broadcast messages; on arrival, symmetric passive association is created, looks at source; if source at higher stratum, reply and terminate; otherwise, reply, and source synchronizes itself to peer. Also, allow control messages.
    5. Selection of source peer, smoothing of data: retains last 8 values of (delay, clock offset), and chooses point with lowest delay, and sets dispersion. Best source chosen from the sets (one per possible source).
    6. Packet receipt, sending: on receipt, if connections compatible, checks not a duplicate by looking at transmit times, checks the last packet received by peer was the last one sent; on failure, set sanity check but continue. Update association to reflect data in newly-0arrived packet; check peer clock, stratum level, validate 2-way communication. If sanity check set, exit. Else estimate delay, clock offset, dispersion, and update local clock.
    7. Delay compensation: statistical in nature, calculates delay and clock offset relative to peer
    8. Access Control: trusted (can synchronize to), friendly (can synchronize), all others (ignore) -- relies on unauthenticated source information in packet
    9. Authentication: optional, uses pairwise secret keys. Authenticator excluded from integrity checking; no key distribution mechanism.

Notes taken by Fritz Barnes [PS] [TEX] [TXT]
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Department of Computer Science
University of California at Davis
Davis, CA 95616-8562

Page last modified on 6/4/97