from Alice's
Adventures in Wonderland, Lewis Carroll
The rabbit toots to make sure you read this.
This document was last updated 7 July 2001
This release of the NTP Version 4 (NTPv4) daemon for Unix, VMS and Windows (NT4 and 2000) incorporates new features and refinements to the NTP Version 3 (NTPv3) algorithms. However, it continues the tradition of retaining backwards compatibility with older versions, including NTPv3 and NTPv2, but not NTPv1. Support for NTPv1 has been discontinued because of certain security vulnerabilities. The NTPv4 version has been under development for quite a while and isn't finished yet. In fact, quite a number of NTPv4 features have already been retrofitted in the older NTPv3, although this version is not actively maintained by the NTPv4 developer's group.
The code compiles and runs properly in at the test host configuration available to the developer corps, including Sun Microsystems, Digital/Compaq, Hewlett Packard, FreeBSD and Linux. Other volunteers have verified it works in IRIX, Windows NT and several others. We invite comments and corrections about the various architectures, operating systems and hardware complement that can't be verified by the developer corps. Of particular interest are Windows 2000, VMS and various reference clock drivers. As always, corrections and bugfixes are warmly received, especially in the form of context diffs sent to <bugs@ntp.org><.
This note summarizes the differences between this software release of NTPv4, called ntp-4.x.x, and the previous NTPv3 version, called xntp3-5.x.x. Additional information on protocol compatibility details is in the Protocol Conformance Statement page.
Most calculations are now done using 64-bit floating double format, rather than 64-bit fixed point format. The motivation for this is to reduce size, improve speed and avoid messy bounds checking. Workstations of today are much faster than when the original NTP version was designed in the early 1980s, and it is rare to find a processor architecture that does not support floating double. The fixed point format is still used with raw timestamps, in order to retain the full precision of about 212 picoseconds. However, the algorithms which process raw timestamps all produce fixed point differences before converting to floating double. The differences are ordinarily quite small so can be expressed without loss of accuracy in this format.
The clock discipline algorithms have been redesigned to improve accuracy, reduce the impact of network jitter and allow increased in poll intervals to well over one day with only moderate sacrifice in accuracy. A new feature called huffpuff maximizes accuracy in cases of highly asymmetric network delays typical of ISDN and modem access circuits. The NTPv4 design allows servers to increase the poll intervals even when synchronized directly to the peer. In NTPv3 the poll interval in such cases was clamped to the minimum, usually 64 s. For those servers with hundreds of clients, the new design can dramatically reduce the network load, especially when large numbers of potential clients, as in national laboratory services. A scheme designed to reduce "clockhopping" when the choice of servers changes frequently as the result of comparatively insignificant quality changes.
This release includes support for the nanokernel precision time kernel support, which is now in stock Linux and FreeBSD kernels. If a precision time source such as a GPS timing receiver or cesium clock is available, kernel timekeeping can be improved to the order of one microsecond. The older precision time kernel for the Digital Tru64 and Ultrix, as well as Sun Microsystems SunOS and Solaris, continues to be supported.
This release includes support for Autokey public-key cryptography, which is the preferred scheme for authenticating servers to clients. It uses NTP header extensions fields based on: Mills, D.L. Public-Key cryptography for the Network Time Protocol. Internet Draft draft-ietf-stime-ntpauth-01.txt, University of Delaware, April 2001, 45 pp. (ASCII). However, this release implements Version 2 of the Autokey protocol, which includes support for additional message digest and digital signature schemes supported by the OpenSSL software library. The new design greatly simplifies key generation and distribution and provides orderly key refreshment. Security procedures and media formats are consistent with industry standard X.509 certificates and authority procedures. Specific improvements to the protocol include a reduction in the number of messages required and a method to protect the cookie used in client/server mode against disclosure. Additional information about Autokey cryptography is contained in the Authentication Options page and links from there. See also the new cryptostats monitoring statistics file in the Monitoring Options page.
NTPv4 includes two new association modes which in most applications can avoid per-host configuration altogether. Both of these are based on IP multicast technology and Autokey cryptography. They provide automatic discovery, configuration and authentication of servers and clients without identifying servers or clients in advance. In multicast mode a server sends a message at fixed intervals using specified multicast group addresses, while clients listen on these addresses. Upon receiving the message, a client exchanges several messages with the server in order to calibrate the multicast propagation delay between the client and server. In manycast mode a client sends a message to a specified multicast group address and expects one or more servers to reply. Using engineered algorithms, the client selects an appropriate subset of servers from the messages received and continues an ordinary client/server campaign. The manycast scheme can provide somewhat better accuracy than the multicast scheme at the price of additional network overhead. See the Association Management page for further information.
There are two 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 reference clock driver interface is smaller, more rational and more accurate. Support for pulse-per-second (PPS) signals has been extended to all drivers as an intrinsic function. Most of the drivers in NTPv3 have been converted to the NTPv4 interface and continue to operate as before. New drivers have been added for several GPS receivers now on the market for a total of 39 drivers. Drivers for the Canadian standard time and frequency station CHU, the US standard time and frequency stations WWV/H and for IRIG signals have been updated and capabilities added to allow direct connection of these signals to the Sun audio port /dev/audio. See the Reference Clock Audio Drivers page for further information.
In all except a very few cases, all timing intervals are randomized, so that the tendency for NTPv3 to self-synchronize and bunch messages, especially with a large number of configured associations, is minimized.
In NTPv3 a large number of weeds and useless code had grown over the years since the original NTPv1 code was implemented almost twenty years ago. Using a powerful weedwacker, much of the shrubbery has been removed, with effect a substantial reduction in size of almost 40 percent.
The entire distribution has been converted to gnu automake, which should greatly ease the task of porting to new and different programming environments, as well as reduce the incidence of bugs due to improper handling of idiosyncratic kernel functions.
There are a few things different about this release that have changed since the latest NTP Version 3 release. Following are a few things to worry about:
As required by Defense Trade Regulations (DTR), the cryptographic routines supporting the Data Encryption Standard (DES) have been removed from the base distribution of NTPv3. For NTPv4 a new interface has been implemented for the OpenSSL cryptographic library, which is widely available on the web at www.openssl.org. This library replaces the library formerly available from RSA Laboratories.. Besides being somewhat faster and widely available, the OpenSSL library supports many additional cryptographic algorithms, which are now selectable at run time. Directions for using OpenSSL are in the Building and Installing the Distribution page.
This version implements Version 2 of the Autokey protocol, which uses packet formats based on the encoding rules of ASN.1. Therefore, it is not backwards comaptible with previous versions of the Autokey protocol. The new version uses standard cryptographic keys and X.509 certificates produced by NTP utility programs, the OpenSSL support services and commercial certificate authorities. In addition, this version provides additional security protection for the cookies used in client/server modes.
Support for the NTPv1 symmetric mode has been discontinued, since it hasn't worked for years. Support continues for the NTPv1 client mode, which is used in some SNTP clients.
As the result of the above, the ./authstuff directory, intended as a development and testing aid for porting cryptographic routines to exotic architectures, has been removed. Testing and conformance validation tools are in the OpenSSL software distrbution.
The NTPv4 enable and disable commands have a few changes in the arguments. See the ntpd Configuration Options page for details. Note that the authenticate command has been removed.
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. It is possible to operate the daemon 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.
To help reduce the level of spurious network traffic due to obsolete configuration files, a special control message called the kiss-of-death packet has been implemented. If enabled and a packet is denied service or exceeds the client limits, a compliant server will send this message to the client. A compliant client will cease further transmission and send a message to the system log. See the Authentication Options page for further information.
An experimental filter algorithm called huff-n'-puff has been implemented to reduce errors under conditions of severe assymetric delays characteristic of ppp connections with telephone modems and downloading or uploading considerable traffic. See the ntpd - Network Time Protocol (NTP) daemon page for further information.
This release has been compiled and tested on many systems, including SunOS 4.1.3, Solaris 2.5.1-2.8, Alpha Tru64 4.0, Ultrix 4.4, Linux 2.4.2, FreeBSD 4.3 and HP-UX 10.02. It has been compiled and tested on Windows NT, but not yet on any other Windows version or for VMS. There are several new features apparently incompatible with Linux systems, including some modes used with the Autokey protocol. The developers corps looks for help elsewhere to resolve these differences. We are relying on the NTP volunteer corps to do that. Known problems are summarized below:
The latest NTPv4 ntpdc does not work with previous versions of ntpd and previous versions of ntpdc do not work with latest ntpd. This situation is regrettable and may be fixed in future; however, it is necessary in order for the autokey function to retrieve canonical names and certificates from directory services such as Secure DNS.
The tty_clk and ppsclock pulse-per-second (PPS) line discipline/streams modules are no longer supported. The PPS function is now handled by the PPS Clock Discipline driver, which uses the new PPSAPI application program interface proposed by the IETF. Note that the pps configuration file command has been obsoleted by the driver. See the Pulse-per-second (PPS) Signal Interfacing page for further information.
The precision time support in stock Solaris 2.6 has bugs that were fixed in 2.7. A patch is available that fixes the 2.6 bugs. The 2.6 PPS kernel discipline has been disabled by default. For testing, the kernel can be enabled using the enable kernel command either in the configuration file or via ntpdc.
The HTML documentation has been partially updated. However, most of the NTPv3 documentation continues to apply to NTPv4. Until a comprehensive update happens, what you see is what you get. We are always happy to accept comments, corrections and bug reports. However, we are most thrilled upon receipt of patches to fix the dang bugs, but please send bug reports to bugs@ntp.org, not the individual members on the team.
David L. Mills
<mills@udel.edu>