Торлар арасында уақытты синхрондау протоколы

Standard protocol for synchronizing time across devices

The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet switched, variable latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in current use. NTP was designed by David L. Mills of the University of Delaware. NTP is intended to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time (UTC). The protocol is usually described in terms of a client–server model, but can as easily be used in peer to peer relationships where both peers consider the other to be a potential time source. They can also use broadcasting or multicasting, where clients passively listen to time updates after an initial round trip calibrating exchange. and a public protocol was developed from it that was documented in The technology was first deployed in a local area network as part of the Hello routing protocol and implemented in the Fuzzball router, an experimental operating system used in network prototyping, where it ran for many years. Other related network tools were available both then and now. They include the Daytime and Time protocols for recording the time of events, as well as the ICMP Timestamp messages and IP Timestamp option More complete synchronization systems, although lacking NTP's data analysis and clock disciplining algorithms, include the Unix daemon timed, which uses an election algorithm to appoint a server for all the clients; and the Digital Time Synchronization Service (DTSS), which uses a hierarchy of servers similar to the NTP stratum model. In 1985, NTP version 0 (NTPv0) was implemented in both Fuzzball and Unix, and the NTP packet header and round trip delay and offset calculations, which have persisted into NTPv4, were documented in Despite the relatively slow computers and networks available at the time, accuracy of better than 100 milliseconds was usually obtained on Atlantic spanning links, with accuracy of tens of milliseconds on Ethernet networks. In 1988, a much more complete specification of the NTPv1 protocol, with associated algorithms, was published in It drew on the experimental results and clock filter algorithm documented in and was the first version to describe the client–server and peer to peer modes. In 1991, the NTPv1 architecture, protocol and algorithms were brought to the attention of a wider engineering community with the publication of an article by David L. Mills in the IEEE Transactions on Communications. In 1989, was published defining NTPv2 by means of a state machine, with pseudocode to describe its operation. It introduced a management protocol and cryptographic authentication scheme which have both survived into NTPv4, along with the bulk of the algorithm. However the design of NTPv2 was criticized for lacking formal correctness by the DTSS community, and the clock selection procedure was modified to incorporate Marzullo's algorithm for NTPv3 onwards. In 1992, defined NTPv3. The RFC included an analysis of all sources of error, from the reference clock down to the final client, which enabled the calculation of a metric that helps choose the best server where several candidates appear to disagree. Broadcast mode was introduced. In subsequent years, as new features were added and algorithm improvements were made, it became apparent that a new protocol version was required. In 2010, was published containing a proposed specification for NTPv4. Following the retirement of Mills from the University of Delaware, the reference implementation is currently maintained as an open source project led by Harlan Stenn. On the IANA side, a ntp (network time protocols) work group is in charge of reviewing proposed drafts. The protocol has significantly progressed since NTPv4.

As NTP replaced the use of the old Time Protocol, some use cases nevertheless found the full protocol too complicated. In 1992, Simple Network Time Protocol (SNTP) was defined to fill this niche. The SNTPv3 standard describes a way to use NTPv3, such that no storage of state over extended periods of time is needed. The topology becomes essentially the same as with the Time Protocol, as only one server is used. In 1996, SNTP was updated to SNTPv4 with some features of the then in development NTPv4. The current version of SNTPv4 is merged into the main NTPv4 standard in 2010. SNTP is fully interoperable with NTP since it does not define a new protocol. However, the simple algorithms provide times of reduced accuracy and thus it is inadvisable to sync time from an SNTP source.

The 64 bit binary fixed point timestamps used by NTP consist of a 32 bit part for seconds and a 32 bit part for fractional second, giving a time scale that rolls over every 232 seconds (136 years) and a theoretical resolution of 2−32 seconds (233 picoseconds). NTP uses an epoch of January 1, 1900. Therefore, the first rollover occurs on February 7, 2036. NTPv4 introduces a 128 bit date format: 64 bits for the second and 64 bits for the fractional second. The most significant 32 bits of this format is the Era Number which resolves rollover ambiguity in most cases. According to Mills, "The 64 bit value for the fraction is enough to resolve the amount of time it takes a photon to pass an electron at the speed of light. The 64 bit second value is enough to provide unambiguous time representation until the universe goes dim."

The NTP reference implementation, along with the protocol, has been continuously developed for over 20 years. Backwards compatibility has been maintained as new features have been added. It contains several sensitive algorithms, especially to discipline the clock, that can misbehave when synchronized to servers that use different algorithms. The software has been ported to almost every computing platform, including personal computers. It runs as a daemon called ntpd under Unix or as a service under Windows. Reference clocks are supported and their offsets are filtered and analysed in the same way as remote servers, although they are usually polled more frequently.

All Microsoft Windows versions since Windows 2000 include the Windows Time service (W32Time), which has the ability to synchronize the computer clock to an NTP server. W32Time was originally implemented for the purpose of the Kerberos version 5 authentication protocol, which required time to be within 5 minutes of the correct value to prevent replay attacks. The network time server in Windows 2000 Server (and Windows XP) does not implement NTP disciplined synchronization, only locally disciplined synchronization with NTP/SNTP correction. Beginning with Windows Server 2003 and Windows Vista, the NTP provider for W32Time became compatible with a significant subset of NTPv3. Microsoft states that W32Time cannot reliably maintain time synchronization with one second accuracy. If higher accuracy is desired, Microsoft recommends using a newer version of Windows or different NTP implementation. Beginning with Windows 10 version 1607 and Windows Server 2016, W32Time can be configured to reach time accuracy of 1 s, 50 ms or 1 ms under certain specified operating conditions.

In 2004, Henning Brauer of OpenBSD presented OpenNTPD, an NTPv3/SNTPv4 implementation with a focus on security and encompassing a privilege separated design. Whilst it is aimed more closely at the simpler generic needs of OpenBSD users, it also includes some protocol security improvements while still being compatible with existing NTP servers. The simpler code base sacrifices accuracy, deemed unnecessary in this use case. A portable version is available in Linux package repositories.

NTPsec is a fork of the reference implementation that has been systematically security hardened. The fork point was in June 2015 and was in response to a series of compromises in 2014. The first production release shipped in October 2017. Between removal of unsafe features, removal of support for obsolete hardware, and removal of support for obsolete Unix variants, NTPsec has been able to pare away 75% of the original codebase, making the remainder easier to audit. A 2017 audit of the code showed eight security issues, including two that were not present in the original reference implementation, but NTPsec did not suffer from eight other issues that remained in the reference implementation.

chrony is an independent NTP implementation mainly sponsored by Red Hat, who uses it as the default time program in their distributions. Being written from scratch, chrony has a simpler codebase allowing for better security and lower resource consumption. Chrony has been evaluated as "trustworthy", with only a few incidents. It is able to achieve improved precision on LAN connections, using hardware timestamping on the network adapter. Support for Network Time Security (NTS) was added on version 4.0. is available under GNU General Public License version 2, was created by Richard Curnow in 1997 and is currently maintained by Miroslav Lichvar.

was started by Poul Henning Kamp of FreeBSD in 2014 and abandoned in 2015. The implementation was sponsored by the Linux Foundation. is the SNTP client built into systemd. It is used by Debian since version "bookworm" and the downstream Ubuntu.

On the day of a leap second event, ntpd receives notification from either a configuration file, an attached reference clock, or a remote server. Although the NTP clock is actually halted during the event, because of the requirement that time must appear to be strictly increasing, any processes that query the system time cause it to increase by a tiny amount, preserving the order of events. If a negative leap second should ever become necessary, it would be deleted with the sequence 23:59:58, 00:00:00, skipping 23:59:59. An alternative implementation, called leap smearing, consists in introducing the leap second incrementally during a period of 24 hours, from noon to noon in UTC time. This implementation is used by Google (both internally and on their public NTP servers), Amazon AWS, and Facebook. Chrony supports leap smear in and configurations, but such use is not to be mixed with a public NTP pool as leap smear is non standard and will throw off client calculation in a mix.

Because adjusting system time is generally a privileged operation, part or all of NTP code has to be run with some privileges in order to support its core functionality. Only a few other security problems have been identified in the reference implementation of the NTP codebase, but those that appeared in 2009 were cause for significant concern. The protocol has been undergoing revision and review throughout its history. The codebase for the reference implementation has undergone security audits from several sources for several years. A stack buffer overflow exploit was discovered and patched in 2014. Apple was concerned enough about this vulnerability that it used its auto update capability for the first time. On systems using the reference implementation, which is running with root user's credential, this could allow unlimited access. Some other implementations, such as OpenNTPD, have smaller code base and adopted other mitigation measures like privilege separation, are not subject to this flaw. A 2017 security audit of three NTP implementations, conducted on behalf of the Linux Foundation's Core Infrastructure Initiative, suggested that both NTP and NTPsec were more problematic than Chrony from a security standpoint. NTP servers can be susceptible to man in the middle attacks unless packets are cryptographically signed for authentication. The computational overhead involved can make this impractical on busy servers, particularly during denial of service attacks. NTP message spoofing from a man in the middle attack can be used to alter clocks on client computers and allow a number of attacks based on bypassing of cryptographic key expiration. Some of the services affected by fake NTP messages identified are TLS, DNSSEC, various caching schemes (such as DNS cache), Border Gateway Protocol (BGP), Bitcoin and a number of persistent login schemes. NTP has been used in distributed denial of service attacks. A small query is sent to an NTP server with the return IP address spoofed to be the target address. Similar to the DNS amplification attack, the server responds with a much larger reply that allows an attacker to substantially increase the amount of data being sent to the target. To avoid participating in an attack, NTP server software can be upgraded or servers can be configured to ignore external queries.

NTP itself includes support for authenticating servers to clients. NTPv3 supports a symmetric key mode, which is not useful against MITM. The public key system known as "autokey" in NTPv4 adapted from IPSec offers useful authentication, with no correction published, save for a change in the message authentication code. Network Time Security (NTS) is a secure version of NTPv4 with TLS and AEAD. The main improvement over previous attempts is that a separate "key establishment" server handles the heavy asymmetric cryptography, which needs to be done only once. If the server goes down, previous users would still be able to fetch time without fear of MITM. NTS is currently supported by several time servers, including Cloudflare. It is supported by NTPSec and chrony. Microsoft also has an approach to authenticate NTPv3/SNTPv4 packets using a Windows domain identity, known as MS SNTP. This system is implemented in the reference ntpd and chrony, using samba for the domain connection.