NTP协议详解：网络时间同步与分布式时钟

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"NTP（网络时间协议）是互联网上的一个标准协议，用于在大型、多样化的网络中同步时间和协调时间分布。它采用可回溯时间设计，通过自组织的主从结构时间服务器子网，与国家时间标准进行同步，并通过有线或无线方式将本地时钟同步到子网内。此外，服务器还可以通过本地路由算法和时间守护进程重新分布参考时间。" 本文档详细介绍了NTP协议，包括其正式结构，并总结了实施过程中有用的信息。NTP协议的核心目标是在互联网中提供一种机制，以确保不同设备的时间同步，这对于网络操作的准确性至关重要。时间同步不仅涉及到日常的计时需求，还支持高速通信和精确时间戳的应用，例如在金融交易、分布式系统和网络安全中。 NTP协议采用了一种容错架构，保证在网络出现故障时仍能保持时间同步。它使用最大似然估计法来校准本地振荡器，这是一种统计方法，用于从多个来源的数据中确定最可能的真实时间值。这种设计允许NTP在可能存在不准确或延迟的网络环境中依然保持高精度。协议中的时间服务器分为不同的层次，形成了一个层次化的主从结构。顶层服务器通常直接与国际原子钟或者卫星信号等高精度时间源连接，向下层服务器提供时间参考。下层服务器则通过网络连接，向更广泛的客户端提供服务。这种分层结构有助于减轻顶层服务器的负载，并且增强了系统的整体稳定性。在NTP的实现中，时间同步过程涉及到复杂的同步算法，如瞬时同步（瞬时对齐）、漂移补偿和抖动控制等，这些算法确保了即使在网络条件变化的情况下，也能提供稳定的时间服务。此外，NTP协议还支持时间戳的交换，使得网络事件可以精确地按时间顺序记录，这对于事件分析和日志记录非常重要。 NTP协议的标准状态由IAB（互联网架构板）维护，并且根据"IAB官方协议标准"的最新版确定其标准化状态和进度。这篇RFC文档不仅是技术规格的描述，也是社区讨论和改进建议的平台。任何感兴趣的个人或组织都可以不受限制地获取和使用NTP协议的文档。 NTP协议是网络时间同步的关键工具，它的存在保证了全球互联网上的设备能够共享一个准确的时间源，从而确保了各种依赖时间的网络服务的正常运行。无论是数据中心的服务器集群，还是家用设备，都受益于NTP提供的准确时间同步功能。

may occupy the full field width with an implied zero preceding bit zero. Hardware and software

packages designed to work with signed quantities will thus yield surprising results when the most

significant (sign) bit is set. It is suggested that externally derived, unsigned fixed-point quantities

such as timestamps be shifted right one bit for internal use, since the precision represented by the

full field width is seldom justified.

Since NTP timestamps are cherished data and, in fact, represent the main product of the protocol,

a special timestamp format has been established. NTP timestamps are represented as a 64-bit

unsigned fixed-point number, in seconds relative to 0

on 1 January 1900. The integer part is in the

first 32 bits and the fraction part in the last 32 bits. This format allows convenient multiple-precision

arithmetic and conversion to Time Protocol representation (seconds), but does complicate the

conversion to ICMP Timestamp message representation (milliseconds). The precision of this

representation is about 200 picoseconds, which should be adequate for even the most exotic

requirements.

Timestamps are determined by copying the current value of the local clock to a timestamp when

some significant event, such as the arrival of a message, occurs. In order to maintain the highest

accuracy, it is important that this be done as close to the hardware or software driver associated with

the event as possible. In particular, departure timestamps should be redetermined for each link-level

retransmission. In some cases a particular timestamp may not be available, such as when the host

is rebooted or the protocol first starts up. In these cases the 64-bit field is set to zero, indicating the

value is invalid or undefined.

Note that since some time in 1968 the most significant bit (bit 0 of the integer part) has been set and

that the 64-bit field will overflow some time in 2036. Should NTP be in use in 2036, some external

means will be necessary to qualify time relative to 1900 and time relative to 2036 (and other

multiples of 136 years). Timestamped data requiring such qualification will be so precious that

appropriate means should be readily available. There will exist an 200-picosecond interval,

henceforth ignored, every 136 years when the 64-bit field will be zero and thus considered invalid.

3.2. State Variables and Parameters

Following is a summary of the various state variables and parameters used by the protocol. They

are separated into classes of system variables, which relate to the operating system environment and

local-clock mechanism; peer variables, which represent the state of the protocol machine specific

to each peer; packet variables, which represent the contents of the NTP message; and parameters,

which represent fixed configuration constants for all implementations of the current version. For

each class the description of the variable is followed by its name and the procedure or value which

controls it. Note that variables are in lower case, while parameters are in upper case. Additional

details on formats and use are presented in later sections and Appendices.

3.2.1. Common Variables

The following variables are common to two or more of the system, peer and packet classes.

Additional variables are specific to the optional authentication mechanism as described in Appendix

RFC-1305 Network Time Protocol (Version 3) March 1992

Mills Page 9

C. When necessary to distinguish between common variables of the same name, the variable

identifier will be used.

Peer Address (peer.peeraddr, pkt.peeraddr), Peer Port (peer.peerport, pkt.peerport): These are the

32-bit Internet address and 16-bit port number of the peer.

Host Address (peer.hostaddr, pkt.hostaddr), Host Port (peer.hostport, pkt.hostport): These are the

32-bit Internet address and 16-bit port number of the host. They are included among the state

variables to support multi-homing.

Leap Indicator (sys.leap, peer.leap, pkt.leap): This is a two-bit code warning of an impending leap

second to be inserted in the NTP timescale. The bits are set before 23:59 on the day of insertion

and reset after 00:00 on the following day. This causes the number of seconds (rollover interval)

in the day of insertion to be increased or decreased by one. In the case of primary servers the

bits are set by operator intervention, while in the case of secondary servers the bits are set by

the protocol. The two bits, bit 0 and bit 1, respectively, are coded as follows:

00 no warning

01 last minute has 61 seconds

10 last minute has 59 seconds

11 alarm condition (clock not synchronized)

In all except the alarm condition (11

), NTP itself does nothing with these bits, except pass them

on to the time-conversion routines that are not part of NTP. The alarm condition occurs when,

for whatever reason, the local clock is not synchronized, such as when first coming up or after

an extended period when no primary reference source is available.

Mode (peer.mode, pkt.mode): This is an integer indicating the association mode, with values coded

as follows:

0 unspecified

1 symmetric active

2 symmetric passive

3 client

4 server

5 broadcast

6 reserved for NTP control messages

7 reserved for private use

Stratum (sys.stratum, peer.stratum, pkt.stratum): This is an integer indicating the stratum of the local

clock, with values defined as follows:

0 unspecified

1 primary reference (e.g., calibrated atomic clock, radio clock)

2-255 secondary reference (via NTP)

RFC-1305 Network Time Protocol (Version 3) March 1992

Mills Page 10

For comparison purposes a value of zero is considered greater than any other value. Note that

the maximum value of the integer encoded as a packet variable is limited by the parameter

NTP.MAXSTRATUM.

Poll Interval (sys.poll, peer.hostpoll, peer.peerpoll, pkt.poll): This is a signed integer indicating the

minimum interval between transmitted messages, in seconds as a power of two. For instance, a

value of six indicates a minimum interval of 64 seconds.

Precision (sys.precision, peer.precision, pkt.precision): This is a signed integer indicating the

precision of the various clocks, in seconds to the nearest power of two. The value must be

rounded to the next larger power of two; for instance, a 50-Hz (20 ms) or 60-Hz (16.67 ms)

power-frequency clock would be assigned the value -5 (31.25 ms), while a 1000-Hz (1 ms)

crystal-controlled clock would be assigned the value -9 (1.95 ms).

Root Delay (sys.rootdelay, peer.rootdelay, pkt.rootdelay): This is a signed fixed-point number

indicating the total roundtrip delay to the primary reference source at the root of the synchroni-

zation subnet, in seconds. Note that this variable can take on both positive and negative values,

depending on clock precision and skew.

Root Dispersion (sys.rootdispersion, peer.rootdispersion, pkt.rootdispersion): This is a signed

fixed-point number indicating the maximum error relative to the primary reference source at the

root of the synchronization subnet, in seconds. Only positive values greater than zero are

possible.

Reference Clock Identifier (sys.refid, peer.refid, pkt.refid): This is a 32-bit code identifying the

particular reference clock. In the case of stratum 0 (unspecified) or stratum 1 (primary reference

source), this is a four-octet, left-justified, zero-padded ASCII string, for example (see Appendix

A for comprehensive list):

Stratum Code Meaning

0 DCN DCN routing protocol

0 TSP TSP time protocol

1 ATOM Atomic clock (calibrated)

1 WWVB WWVB LF (band 5) radio

1 GOES GOES UHF (band 9) satellite

1 WWV WWV HF (band 7) radio

In the case of stratum 2 and greater (secondary reference) this is the four-octet Internet address

of the peer selected for synchronization.

Reference Timestamp (sys.reftime, peer.reftime, pkt.reftime): This is the local time, in timestamp

format, when the local clock was last updated. If the local clock has never been synchronized,

the value is zero.

RFC-1305 Network Time Protocol (Version 3) March 1992

Mills Page 11

Originate Timestamp (peer.org, pkt.org): This is the local time, in timestamp format, at the peer

when its latest NTP message was sent. If the peer becomes unreachable the value is set to zero.

Receive Timestamp (peer.rec, pkt.rec): This is the local time, in timestamp format, when the latest

NTP message from the peer arrived. If the peer becomes unreachable the value is set to zero.

Transmit Timestamp (peer.xmt, pkt.xmt): This is the local time, in timestamp format, at which the

NTP message departed the sender.

3.2.2. System Variables

Table 1 shows the complete set of system variables. In addition to the common variables described

previously, the following variables are used by the operating system in order to synchronize the

local clock.

Local Clock (sys.clock): This is the current local time, in timestamp format. Local time is derived

from the hardware clock of the particular machine and increments at intervals depending on the

design used. An appropriate design, including slewing and skew-Compensation mechanisms,

is described in Section 5.

Clock Source (sys.peer): This is a selector identifying the current synchronization source. Usually

this will be a pointer to a structure containing the peer variables. The special value NULL

indicates there is no currently valid synchronization source.

3.2.3. Peer Variables

Table 2 shows the complete set of peer variables. In addition to the common variables described

previously, the following variables are used by the peer management and measurement functions.

Configured Bit (peer.config): This is a bit indicating that the association was created from

configuration information and should not be demobilized if the peer becomes unreachable.

System Variables Name Procedure

Leap Indicator sys.leap clock update

Stratum sys.stratum clock update

Precision sys.precision system

Root Delay sys.rootdelay clock update

Root Dispersion sys.rootdispersion clock update

Reference Clock Ident sys.refid clock update

Reference Timestamp sys.reftime clock update

Local Clock sys.clock clock update

Clock Source sys.peer selection

Poll Interval sys.poll local clock

Cryptographic Keys sys.keys authentication

Table 1. System Variables

RFC-1305 Network Time Protocol (Version 3) March 1992

Mills Page 12

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