About NTP Version 4

NTP has been under development for almost 30 years, but the paint ain't dry even now. This release of the NTP Version 4 (NTPv4) distribution for Unix, VMS and Windows incorporates new features and refinements, but retaining backwards compatibility with older versions, including NTPv3 and NTPv2, but not NTPv1. Support for NTPv1 has been discontinued because of certain security vulnerabilities.

New Features

The behavior of the daemon at startup has been considerably improved. The time to measure the frequency and correct an initial offset error when started for the first time is now no more than ten minutes. Upon restart, it takes no more than five minutes to reduce the initial offset to less than one millisecond without adversely affecting the frequency. This avoids a subsequent frequency correction which could take up to several hours.
A new feature called interleaved mode can be used in NTP symmetric and broadcast modes. It is designed to improve accuracy by minimizing errors due to queuing and transmission delays. It is described on the NTP Interleaved Modes page.
The huff-n'-puff filter is designed to avoid large errors with DSL circuits and highly asymmetrical traffic, as when downloading large files. Details are on the The Huff-n'-Puff Filter page.
A new feature called orphan mode provides an automatic, subnet-wide synchronization feature with multiple sources. It provides reliable backup in isolated networks or in pr when Internet sources have become unavailable. See the Orphan Mode page for further information.

This release includes comprehensive packet rate management tools to help reduce the level of spurious network traffic and protect the busiest servers from overload. There is support for the optional Kiss-o'-Death (KoD) packet intended to slow down an abusive client. See the Rate Management and the Kiss-o'-Death Packet page for further information.

There are two new burst mode features available where special conditions apply. One of these is enabled by the iburst keyword in the server configuration command. It is intended for cases where it is important to set the clock quickly when an association is first mobilized. The other is enabled by the burst keyword in the server configuration command. It is intended for cases where the network attachment requires an initial calling or training procedure. See the Association Management page for further information.
The OpenSSL cryptographic library has replaced the library formerly available from RSA Laboratories. All cryptographic routines except a version of the MD5 message digest algorithm have been removed from the base distribution. All 128-bit and 160-bit message digests algorithms are now supported for both symmetric key and public key cryptosystems. See the Authentication Support page for further information and the Authentication Options page for a list of supported digest algorithms.
This release includes support for Autokey public-key cryptography for authenticating public servers to clients, as described in RFC 5906. This support requires the --enable-autokey option when building the distribution, which is the default is OpenSSL is available. The deployment of Autokey subnets is now considerably simpler than in earlier versions. A subnet naming scheme is now available to filter manycast and pool configurations. Additional information about Autokey is on the Autokey Public Key Authentication page and links from there.

The NTP descrete even simulator has been substantially upgraded, now including scenarios with multiple servers and time-sensitive scripts. This allows the NTP algorithms to be tested in an embedded environment with systematic and pseudo-random network delay and oscillator wander distributions. This has been used to verify correct operation under conditions of extreme error and misconfiguration. See the ntpdsim - Network Time Protocol (NTP) simulator page. A technical description and performance analysis is given in the white papers at the NTP Project Page.

NTPv4 includes three new server discovery schemes, which in most applications can avoid per-host configuration altogether. Two of these are based on IP multicast technology, while the remaining one is based on crafted DNS lookups. See the Automatic NTP Configuration Schemes page for further information.

The status display and event report monitoring functions have been considerably expanded, including new statistics files and event reporting to files and the system log. See the Event Messages and Status Words page for further information.
Several new options have been added for the ntpd command line. For the inveterate knob twiddlers several of the more important performance variables can be changed to fit actual or perceived special conditions. In particular, the tinker and tos commands can be used to adjust thresholds, throw switches and change limits.

The ntpd daemon can be operated in a one-time mode similar to ntpdate, which program is headed for retirement. See the ntpd - Network Time Protocol (NTP) daemon page for the new features.