Windows Time Client.


Appendix D

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Differences from Previous Versions

 

The original NTP, later called NTP Version 0, was described in RFC-958 [MIL85c]. Subsequently, Version 0 was superseded by Version 1 (RFC-1059 [MIL88a]), and Version 2 (RFC-1119 [MIL89]. The Version-2 description was split into two documents, RFC-1119 defining the architecture and specifying the protocol and algorithms, and another [MIL90b] describing the service model, algorithmic analysis and operating experience. In previous versions these two objectives were combined in one document. While the architecture assumed in Version 3 is identical to Version 2, the protocols and algorithms differ in minor ways. Differences between NTP Version 3 and previous versions are described in this Appendix. Due to known bugs in very old implementations, continued support for Version-0 implementations is not recommended. It is recommended that new implementations follow the guidelines below when interoperating with older implementations.

 

Version 3 neither changes the protocol in any significant way nor obsoletes previous versions or existing implementations. The main motivation for the new version is to refine the analysis and implementation models for new applications at much higher network speeds to the gigabit-per-second regime and to provide for the enhanced stability, accuracy and precision required at such speeds. In particular, the sources of time and frequency errors have been rigorously examined and error bounds established in order to improve performance, provide a model for correctness assertions and indicate timekeeping quality to the user. Version 3 also incorporates two new optional features, (1) an algorithm to combine the offsets of a number of peer time servers in order to enhance accuracy and (2) improved local-clock algorithms which allow the poll intervals on all synchronization paths to be substantially increased in order to reduce network overhead. Following is a summary of previous versions of the protocol together with details of the Version 3 changes.

 

1. Version 1 supports no modes other than symmetric-active and symmetric-passive, which are determined by inspecting the port-number fields of the UDP packet header. The peer mode can be determined explicitly from the packet-mode variable (pkt.mode) if it is nonzero and implicitly from the source port (pkt.peerport) and destination port (pkt.hostport) variables if it is zero. For the case where pkt.mode is zero the mode is determined as follows:

 

pkt.peerport

pkt.hostport

Mode

NTP.PORT

NTP.PORT

symmetric active

NTP.PORT

not NTP.PORT

server

not NTP.PORT

NTP.PORT

client

not NTP.PORT

not NTP.PORT

not possible

 

Note that it is not possible in this case to distinguish between symmetric active and symmetric passive modes. Use of the pkt.mode and NTP.PORT variables in this way is not recommended and may not be supported in future versions of the protocol. The low-order three bits of the first octet, specified as zero in Version 1, are used for the mode field in Version 2. Version-2 and Version-3 implementations interoperating with Version-1 implementations should operate in a passive mode only and use the value one in the version number (pkt.version) field and zero in the mode (pkt.mode) field in transmitted messages.

 

2. Version 1 does not support the NTP control message described in Appendix B. Certain old versions of the Unix NTP daemon ntpd use the high-order bits of the stratum field (pkt.stratum) for control and monitoring purposes. While these bits are never set during normal Version-1, Version-2 or Version-3 operations, new implementations may use the NTP reserved mode 6 described in Appendix B and/or private reserved mode 7 for special purposes, such as remote control and monitoring, and in such cases the format of the packet following the first octet can be arbitrary. While there is no guarantee that different implementations can interoperate using private reserved mode 7, it is recommended that vanilla ASCII format be used whenever possible.

 

3. Version 1 does not support authentication. The key identifiers, cryptographic keys and procedures described in Appendix C are new to Version 2 and continued in Version 3, along with the corresponding variables, procedures and authenticator fields. In the NTP message described in Appendix A and NTP control message described in Appendix B the format and contents of the header fields are independent of the authentication mechanism and the authenticator itself follows the header fields, so that previous versions will ignore the authenticator.

 

4. In Version 1 the total dispersion (pkt.rootdispersion) field of the NTP header was called the estimated drift rate, but not used in the protocol or timekeeping procedures. Implementations of the Version-1 protocol typically set this field to the current value of the skew-compensation register, which is a signed quantity. In a Version 2 implementation apparent large values in this field may affect the order considered in the clock-selection procedure. Version-2 and Version-3 implementations interoperating with older implementations should assume this field is zero, regardless of its actual contents.

 

5. Version 2 and Version 3 incorporate several sanity checks designed to avoid disruptions due to unsynchronized, duplicate or bogus timestamp information. The checks in Version 3 are specifically designed to detect lost or duplicate packets and resist invalid timestamps. The leap-indicator bits are set to show the unsynchronized state if updates are not received from a reference source for a considerable time or if the reference source has not received updates for a considerable time. Some Version-1 implementations could claim valid synchronization indefinitely following loss of the reference source.

 

6. The clock-selection procedure of Version 2 was considerably refined as the result of accumulated experience with the Version-1 implementation. Additional sanity checks are included for authentication, range bounds and to avoid use of very old data. The candidate list is sorted twice, once to select a relatively few robust candidates from a potentially large population of unruly peers and again to order the resulting list by measurement quality. As in Version 1, The final selection procedure repeatedly casts out outlyers on the basis of weighted dispersion.

 

7. The local-clock procedure of Version 2 were considerably improved over Version 1 as the result of analysis, simulation and experience. Checks have been added to warn that the oscillator has gone too long without update from a reference source. The compliance register has been added to improve frequency stability to the order of a millisecond per day. The various parameters were retuned for optimum loop stability using measured data over typical Internet paths and with typical local-clock hardware. In version 3 the phase-lock loop model was further refined to provide an adaptive-bandwidth feature that automatically adjusts for the inherent stabilities of the reference clock and local clock while providing optimum loop stability in each case.

 

8. Problems in the timekeeping calculations of Version 1 with high-speed LANs were found and corrected in Version 2. These were caused by jitter due to small differences in clock rates and different precisions between the peers. Subtle bugs in the Version-1 reachability and polling-rate control were found and corrected. The peer.valid and sys.hold variables were added to avoid instabilities when the reference source changes rapidly due to large dispersive delays under conditions of severe network congestion. The peer.config, peer.authenable and peer.authentic bits were added to control special features and simplify configuration.

 

9. In Version 3 The local-clock algorithm has been overhauled to improve stability and accuracy. Appendix G presents a detailed mathematical model and design example which has been refined with the aid of feedback-control analysis and extensive simulation using data collected over ordinary Internet paths. Section 5 of RFC-1119 on the NTP local clock has been completely rewritten to describe the new algorithm. Since the new algorithm can result in message rates far below the old ones, it is highly recommended that they be used in new implementations. Note that this algorithm is not integral to the NTP protocol specification itself and its use does not affect interoperability with previous versions or existing implementations; however, in order to insure overall NTP subnet stability in the Internet, it is essential that the local-clock characteristics of all NTP time servers conform to the analytical models presented previously and in this document.

 

10. In Version 3 a new algorithm to combine the offsets of a number of peer time servers is presented in Appendix F. This algorithm is modelled on those used by national standards laboratories to combine the weighted offsets from a number of standard clocks to construct a synthetic laboratory time scale more accurate than that of any clock separately. It can be used in an NTP implementation to improve accuracy and stability and reduce errors due to asymmetric paths in the Internet. The new algorithm has been simulated using data collected over ordinary Internet paths and, along with the new local-clock algorithm, implemented and tested in the Fuzzball time servers now running in the Internet. Note that this algorithm is not integral to the NTP protocol specification itself and its use does not affect interoperability with previous versions or existing implementations.

 

11. Several inconsistencies and minor errors in previous versions have been corrected in Version 3. The description of the procedures has been rewritten in pseudo-code augmented by English commentary for clarity and to avoid ambiguity. Appendix I has been added to illustrate C-language implementations of the various filtering and selection algorithms suggested for NTP. Additional information is included in Section 5 and in Appendix E, which includes the tutorial material formerly included in Section 2 of RFC-1119, as well as much new material clarifying the interpretation of timescales and leap seconds.

 

12. Minor changes have been made in the Version-3 local-clock algorithms to avoid problems observed when leap seconds are introduced in the UTC time scale and also to support an auxiliary precision oscillator, such as a cesium clock or timing receiver, as a precision time base. In addition, changes were made to some procedures described in Section 3 and in the clock-filter and clock-selection procedures described in Section 4. While these changes were made to correct minor bugs found as the result of experience and are recommended for new implementations, they do not affect interoperability with previous versions or existing implementations in other than minor ways (at least until the next leap second).

 

13. In Version 3 changes were made to the way delay, offset and dispersion are defined, calculated and processed in order to reliably bound the errors inherent in the time-transfer procedures. In particular, the error accumulations were moved from the delay computation to the dispersion computation and both included in the clock filter and selection procedures. The clock-selection procedure was modified to remove the first of the two sorting/discarding steps and replace with an algorithm first proposed by Marzullo and later incorporated in the Digital Time Service. These changes do not significantly affect the ordinary operation of or compatibility with various versions of NTP, but they do provide the basis for formal statements of correctness as described in Appendix H.

 


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