Network Time Protocol: A Comprehensive Overview for Efficient Time Services

1. Internet Time Services and NTP, The Network Time Protocol Judah Levine Time and Frequency Division NIST Boulder jlevine@boulder.nist.gov 303 497-3903

2. Components of NTP • Message format and parameters • Measurement protocol • Typical Performance • Traceability • Private NTP • Layered services and authentication • Reference clocks • Peer selection • Clock discipline algorithm • Hierarchy definition and realization • Remote Administration and Monitoring

3. Message Format and Parameters • Format defined in RFC 1305 • Packets are udp/ip • checksum-based error detection • simple connectionless service • no guarantee of delivery • no detection of lost packets • Similar to mailing a letter to a recipient who does not expect it • Message authentication is optional • Authenticator added to message

4. NTP Transport • Independent of physical layer in principle • Performance depends on physical layer in practice • All versions of NTP can use ipV4 protocol • Newer versions also support ipV6 • NIST has 1 ipV6 server (October, 2012)

5. NTP Time Datum • Reference scale is UTC • Time parameters are 64 bits long: Seconds since 1900.0 (32 bits, unsigned) Fraction of a second (32 bits, unsigned) • Dynamic range: 136+ years • rollover in 2036 • Resolution: 2-32 s  232 ps • a time of 0.0 is usually treated as an error

6. Some Other Parameters • Advance notice of leap second • leap second at end of today when set • Positive leap second in progress • Transmit 23:59:59 twice • Effectively stop the system clock • Slewing methods (Google) • Clock unsynchronized flag • don’t trust the time message • Health and leap second use same bits.

7. More Parameters • Message authenticator (optional) • Stratum, precision and reference signal of the server (ref ip or local clock type) • Stratum indicates number of links to UTC, not quality of oscillator • Administrative data (alternate format) • not completely defined in the protocol specification. • Usually can be ignored without problems

8. Authentication - 1 • Keys identified by index number into local file which has the key value. • <Key Number> M <key value> • Configuration • Trustedkey <Key Number> • MD5 hash of the NTP key+packet • XMT: Append result and key index • RCV: Detects additional packet data • Compute hash using key specified by number, compare with received value • No decryption of received message • MD5 algorithm has no known inverse • Distribution of keys is a headache

9. Authentication - 2 • Authentication only identifies server • Guarantees integrity • Does not affect accuracy • Does not affect traceability • Possible “man in the middle” attacks • Delay valid message • Replay old message

10. Two-Way Protocol d Client Server

11. If we assume that Then If the truth is that Then

12. Limits of asymmetry error k=1, =/2 Round-trip delay→  0 k=0, = -/2 Smaller delay has smaller asymmetry error

13. Measurement Protocols • Two-way daemon with measured delay • uses udp/ip port 123 on server and client • Periodic queries to multiple servers • Client-server or peer to peer • Broadcast mode, no measured delay • Single server, multiple clients • Local LAN or Internet multi-cast • Single-shot client, e.g. ntpdate and sntp • port on server is udp/ip port 123 • any port on client is okay • measures delay using usual 2-way method

14. Typical Performance - 1 • Typical asymmetry few percent of delay • Small LAN • 10 ms best case ever on 2-node LAN • 220 ms on real-world small LAN • Typical large-building LAN • 2 ms RMS typical • Internet with few hops • 10 – 20 ms • Long distance and/or slow or busy link • 100 ms – 1 s

15. Typical Performance - 2 • Accuracy determined by accuracy of network delay measurement • not a function of message format • PTP/1588 not better on same network • Accuracy degraded on network with asymmetric traffic delays • Web server • David Mills “huff-n-puff” filter • Minimal delay implies minimal asymmetry

16. Traceability • Log files at stratum-1 server • Positive entries important • NIST servers maintain internal log files of each calibration cycle • Log files at end user necessary and may be sufficient • Queries to multiple servers are useful • Log files shift the legal burden of proof • “best practices” argument • Commercial equipment very poor and inadequate

17. Traceability and private NTP • Private NTP uses special hardware or software at both ends of link • “Trusted Master Clock” at NMI • “Trusted Reference Clock” at end user • Monitoring of user system using separate system • Issuing calibration certificates

18. NIST Position • Time from a private NTP systems is not traceable to NIST even if hardware is at NIST • Traceability ends at the signal connector • NIST does not accept integrity or availability requirements • Some computer security issues not resolved

19. Layered services - 1 • Document time stamping using Public/Private key pair • Hash code of document submitted to NIST • Document itself not transmitted or revealed • NIST adds NTP time stamp and signs combination with private key • Validity and time stamp can be verified using public key • NIST does not have to archive transaction

20. Layered Services - 2 • Certified Mail • NIST receives e-mail message from sender • Message can be encrypted • NIST adds time stamp and signature • NIST forwards mail to recipient • Support for Kerberos ticket system • Time-based method to validate users and applications

21. NIST position • Layered services will be realized by private companies only • NIST service ends at time-service portal • NTP stratum-1 server, ACTS, … • NIST provides authenticated stratum-1 time service to registered users • Registration linked to ip address of user • Unique key for each user

22. References • RFC literature for NTP • 958, 1059, 1119, 1305, 1589, 1769, 2030, ... • Other NTP literature • IEEE/ACM Trans. Networking, Jun 1995, pp. 245-254. • IEEE Trans. Comm., Oct. 1991, pp. 1482-93 • www.ee.udel.edu/~mills/ntp.html • David Mills: mills@udel.edu

23. Other Methods • Lockclock: • IEEE/ACM Trans. Networks, 2/95, pp. 42-50. • Interlock and Autolock: • IEEE Trans. UFFC, 7/99, pp. 888-896. • Other (non-NTP) methods • IEEE Trans. Parallel Distributed Sys., 3/94, pp. 474-487.

24. Backup Slides • Reference Clock Models • Peer Selection Method • Clock Discipline: PLL vs FLL • Polling Interval Considerations • NTP Administration • NTP Message Format and Parameters

25. Reference Clock Models (NTP) • Local reference clock (radio clock ...) incorporated as a peer with a pseudo-ip address (127.127.t.u). • pseudo-ip points to internal driver • Relationship to clock looks like client- server -- clock is polled periodically and time enters standard peer selection procedures.

26. Reference Clock (JL) • Local reference clock is a special device with its own driver • Separate from NTP • statistics tuned to clock type • Better than standard NTP in normal operations • Worse in certain error situations • Longer polling interval

27. NTP Peer Selection - 1 • Initial specification is in configuration file using ip address + keywords • server, peer, prefer • Typical configuration specifies multiple servers • All (or multiple) servers queried on every measurement cycle

28. NTP Peer Selection - 2 • Method favors lowest stratum, lowest distance peer that claims to be healthy and does not appear to be an outlier with respect to the other systems. • Best case • multiple queries improves accuracy by 1/N • Cost increases as N • Statistics of local clock not used • Short-term stability better than network

29. JL Peer management • Algorithm normally queries only one server • Compare received time difference with expected value • Statistics of local clock used in algorithm • Weighted sum of local clock time and time received from remote server • Weighting based on previous variance • Query additional server only if error detected

30. NTP Clock Discipline - I • “Original recipe” NTP uses PLL • Local clock is steered primarily in frequency using filtered time difference. • goal of loop is to drive time error to 0 • approximate second-order loop. • Polling interval must be kept short so that variance is time noise • Discrimination implemented with queries to multiple servers • Cost ~ N, benefit ~ 1/N • loop converts time noise to frequency noise

31. NTP Clock Discipline - II • New version of NTP uses mixed PLL/FLL • crossover as function of interval between measurements and message quality • FLL drives frequency error to 0 • time is correct on the average • defined by Adev • better decoupling of time/frequency

32. JL Clock Discipline • Control loop is pure FLL • parameters set to Adev analysis • Time step detector similar to AT1 • Simplifies separation of time and frequency fluctuations • allows time steps with no associated change in frequency

33. FLL vs. PLL Considerations • Server noise spectrum seen through channel vs. noise of local clock. • local clock often better at short times. • Frequency noise and phase noise are not necessarily correlated • FLL generally cheaper • makes better use of better hardware • FLL slower to remove time steps

34. Cost/Performance tradeoff • A PLL must operate in domain where phase noise is dominant problem • Degradation at longer times is not easily determined • FLL usually operates in white FM domain • Cheaper to start with (longer averaging) • Quantitative tradeoff based on AVAR • Implemented in Autolock

35. Polling Interval • Statistics of remote clock seen through the network should be better than local clock variance • Cost/benefit analysis: • White phase noise: • Cost ~ N, • Variance ~ 1/N

36. Daemon Administration • NTP packet modes 6 and 7 • uses same udp/ip packets to same port • configure daemon parameters • monitor performance and usage • dangerous without authentication • NTP distribution provides utility tasks • ntpq • xntpdc

37. Network Administration • Public servers identified in lists maintained by David Mills at the University of Delaware and ntp.org • RFC literature defines standards, etc. • newsgroup comp.protocols.time.ntp • “Free for all” typical of the Internet • Almost all people are good citizens almost all of the time

38. NIST Server Administration • All network-based administration disabled for security reasons • Each server maintains internal state variable history • Each server linked to alarm system via dial-up modem. • Separate monitoring and alarm system runs on system in Boulder

39. Definition of Stratum • Lower stratum clocks are closer to a primary time source • WWVB, GPS, ... • Properties or quality of local oscillator do not influence stratum definition • NTP message format supports stratum, accuracy estimate and synch. source

40. More Parameters • Message authenticator (optional) • Stratum, precision and reference signal of the server (ref ip or local clock type) • Stratum indicates number of links to UTC, not quality of oscillator • Administrative data (alternate format) • not completely defined in the protocol specification. • Usually can be ignored without problems

41. NTP Service model • Operate servers at many locations • Minimizes delay error for all users • No single point of failure • How are remote servers synchronized? • Time link to source of UTC(lab) • Performance limited by delay jitter and asymmetry • Static asymmetry generally cannot be detected

42. Hierarchy Radio, GPS, cesium, ... 1 1 1 p p c-s p p 2 2 2