Network Working Group D.L. Mills
Request for Comments: 958 M/A-COM Linkabit
September 1985
Network Time Protocol (NTP)
Status of this Memo
This RFC suggests a proposed protocol for the ARPA-Internet
community, and requests discussion and suggestions for improvements.
Distribution of this memo is unlimited.
Table of Contents
1. Introduction
2. Service Model
3. Protocol Overview
4. State Variables and Formats
5. Protocol Operation
5.1. Protocol Modes
5.2. Message Processing
5.3. Network Considerations
5.4. Leap Seconds
6. References
Appendix A. UDP Header Format
Appendix B. NTP Data Format
1. Introduction
This document describes the Network Time Protocol (NTP), a protocol
for synchronizing a set of network clocks using a set of distributed
clients and servers. NTP is built on the User Datagram Protocol
(UDP) [13], which provides a connectionless transport mechanism. It
is evolved from the Time Protocol [7] and the ICMP Timestamp message
[6] and is a suitable replacement for both.
NTP provides the protocol mechanisms to synchronize time in principle
to precisions in the order of nanoseconds while preserving a
non-ambiguous date, at least for this century. The protocol includes
provisions to specify the precision and estimated error of the local
clock and the characteristics of the reference clock to which it may
be synchronized. However, the protocol itself specifies only the
data representation and message formats and does not specify the
synchronizing algorithms or filtering mechanisms.
Other mechanisms have been specified in the Internet protocol suite
to record and transmit the time at which an event takes place,
including the Daytime protocol [8] and IP Timestamp option [9]. The
NTP is not meant to displace either of these mechanisms. Additional
information on network time synchronization can be found in the
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References at the end of this document. An earlier synchronization
protocol is discussed in [3] and synchronization algorithms in [2],
[5], [10] and [12]. Experimental results on measured roundtrip delays
and clock offsets in the Internet are discussed in [4] and [11]. A
comprehensive mathematical treatment of clock synchronization can be
found in [1].
2. Service Model
The intent of the service for which this protocol is designed is to
connect a few primary reference clocks, synchronized by wire or radio
to national standards, to centrally accessable resources such as
gateways. These gateways would use NTP between them to cross-check
the primary clocks and mitigate errors due to equipment or
propagation failures. Some number of local-net hosts, serving as
secondary reference clocks, would run NTP with one or more of these
gateways. In order to reduce the protocol overhead, these hosts
would redistribute time to the remaining local-net hosts. In the
interest of reliability selected hosts might be equipped with less
accurate but less expensive radio clocks and used for backup in case
of failure of the primary and/or secondary clocks or communication
paths between them.
In the normal configuration a subnetwork of primary and secondary
clocks will assume a hierarchical organization with the more accurate
clocks near the top and the less accurate below. NTP provides
information that can be used to organize this hierarchy on the basis
of precision or estimated error and even to serve as a rudimentary
routing algorithm to organize the subnetwork itself. However, the
NTP protocol does not include a specification of the algorithms for
doing this, which is left as a topic for further study.
3. Protocol Overview
There is no provision for peer discovery, acquisition, or
authentication in NTP. Data integrity is provided by the IP and UDP
checksums. No reachability, circuit-management, duplicate-detection
or retransmission facilities are provided or necessary. The service
can operate in a symmetric mode, in which servers and clients are
indistinguishable yet maintain a small amount of state information,
or in an unsymmetric mode in which servers need maintain no client
state other than that contained in the client request. Moreover,
only a single NTP message format is necessary, which simplifies
implementation and can be used in a variety of solicited or
unsolicited polling mechanisms.
In what may be the most common (unsymmetric) mode a client sends an
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NTP message to one or more servers and processes the replies as
received. The server interchanges addresses and ports, fills in or
overwrites certain fields in the message, recalculates the checksum
and returns it immediately. Information included in the NTP message
allows each client/server peer to determine the timekeeping
characteristics of its other peers, including the expected accuracies
of their clocks. Using this information each peer is able to select
the best time from possibly several other clocks, update the local
clock and estimate its accuracy.
It should be recognized that clock synchronization requires by its
nature long periods and multiple comparisons in order to maintain
accurate timekeeping. While only a few comparisons are usually
adequate to maintain local time to within a second, primarily to
protect against broken hardware or synchronization failure, periods
of hours or days and tens or hundreds of comparisons are required to
maintain local time to within a few tens of milliseconds.
Fortunately, the frequency of comparisons can be quite small and
almost always non-intrusive to normal network operations.
4. State Variables and Formats
NTP timestamps are represented as a 64-bit fixed-point number, in
seconds relative to 0000 UT on 1 January 1900. The integer part is
in the first 32 bits and the fraction part in the last 32 bits, as
shown in the following diagram.
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 low-order fraction bit increments at about
0.2-nanosecond intervals, so a free-running one-millisecond clock
will be in error only a small fraction of one part per million, or
less than a second per year.
In some cases a particular timestamp may not be available, such as
when the protocol first starts up. In these cases the 64-bit field
is set to zero, indicating the value is invalid or undefined.
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Following is a description of the various data items used in the
protocol. Details of packet formats are presented in the Appendices.
Leap Indicator
This is a two-bit code warning of an impending leap-second to be
inserted in the internationally coordinated Standard Time
broadcasts. A leap-second is occasionally added or subtracted
from Standard Time, which is based on atomic clocks, to maintain
agreement with Earth rotation. When necessary, the corrections
are notified in advance and executed at the end of the last day of
the month in which notified, usually June or December. When a
correction is executed the first minute of the following day will
have either 59 or 61 seconds.
Status
This is a six-bit code indicating the status of the local clock.
Values are assigned to indicate whether it is operating correctly
or in one of several error states.
Reference Clock Type
This is an eight-bit code identifying the type of reference clock
used to set the local clock. Values are assigned for primary
clocks (locally synchronized to Standard Time), secondary clocks
(remotely synchronized via various network protocols) and even
eyeball-and-wristwatch.
Precision
This is a 16-bit signed integer indicating the precision of the
local clock, in seconds to the nearest power of two. For
instance, a 60-Hz line-frequency clock would be assigned the value
-6, while a 1000-Hz crystal clock would be assigned the value -10.
Estimated Error
This is a 32-bit fixed-point number indicating the estimated error
of the local clock at the time last set. The value is in seconds,
with fraction point between bits 15 and 16, and is computed by the
sender based on the reported error of the reference clock, the
precision and drift rate of the local clock and the time the local
clock was last set. For statistical purposes this quantity can be
assumed equal to the estimated or computed standard deviation, as
described in [12].
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Estimated Drift Rate
This is a 32-bit signed fixed-point number indicating the
estimated drift rate of the local clock. The value is
dimensionless, with fraction point to the left of the high-order
bit. While for most purposes this value can be estimated based on
the hardware characteristics, it is possible to compute it quite
accurately, as described in [12].
Reference Clock Identifier
This is a 32-bit code identifying the particular reference clock.
The interpretation of its value depends on value of Reference
Clock Type. In the case of a primary clock locally synchronized
to Standard Time (type 1), the value is an ASCII string
identifying the clock. In the case of a secondary clock remotely
synchronized to an Internet host via NTP (type 2), the value is
the 32-bit Internet address of that host. In other cases the
value is undefined.
Reference Timestamp
This is a 64-bit timestamp established by the server or client
host as the timestamp (presumably obtained from a reference clock)
most recently used to update the local clock. If the local clock
has never been synchronized, the value is zero.
Originate Timestamp
This is a 64-bit timestamp established by the client host and
specifying the local time at which the request departed for the
service host. It will always have a nonzero value.
Receive Timestamp
This is a 64-bit timestamp established by the server host and
specifying the local time at which the request arrived from the
client host. If no request has ever arrived from the client the
value is zero.
Transmit Timestamp
This is a 64-bit timestamp established by the server host and
specifying the local time at which the reply departed for the
client host. If no request has ever arrived from the client the
value is zero.
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5. Protocol Operation
The intent of this document is to specify a standard for data
representation and message format which can be used for a variety of
synchronizing algorithms and filtering mechanisms. Accordingly, the
information in this section should be considered a guide, rather than
a concise specification. Nevertheless, it is expected that a
standard Internet distributed timekeeping protocol with concisely
specified synchronizing and filtering algorithms can be evolved from
the information in this section.
5.1. Protocol Modes
The distinction between client and server is significant only in
the way they interact in the request/response interchange. The
same NTP message format is used by each peer and contains the same
data relative to the other peer. In the unsymmetric mode the
client periodically sends an NTP message to the server, which then
responds within some interval. Usually, the server simply
interchanges addresses and ports, fills in the required
information and sends the message right back. Servers operating in
the unsymmetric mode then need retain no state information between
client requests.
In the symmetric mode the client/server distinction disappears.
Each peer maintains a table with as many entries as active peers,
each entry including a code uniquely identifying the peer (e.g.
Internet address), together with status information and a copy of
the Originate Timestamp and Receive Timestamp values last received
from that peer. The peer periodically sends an NTP message to each
of these peers including the latest copy of these timestamps. The
interval between sending NTP messages is managed solely by the
sending peer and is unaffected by the arrival of NTP messages from
other peers.
The mode assumed by a peer can be determined by inspection of the
UDP Source Port and Destination Port fields (see Appendix A). If
both of these fields contain the NTP service-port number 123, the
peer is operating in symmetric mode. If they are different and
the Destination Port field contains 123, this is a client request
and the receiver is expected to reply in the manner described
above. If they are different and the Source Port field contains
123, this is a server reply to a previously sent client request.
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5.2. Message Processing
The significant events of interest in NTP occur usually near the
times the NTP messages depart and arrive the client/server. In
order to maintain the highest accuracy it is important that the
timestamps associated with these events be computed as close as
possible to the hardware or software driver associated with the
communications link and, in particular, that departure timestamps
be recomputed for each retransmission, if used at the link level.
An NTP message is constructed as follows (see Appendix B). The
source peer constructs the UDP header and the LI, Status,
Reference Clock Type and Precision fields in the NTP data portion.
Next, it determines the current synchronizing source and
constructs the Type and Reference Clock Identifier fields. From
its timekeeping algorithm (see [12] for examples) it determines
the Reference Timestamp, Estimated Error and Estimated Drift Rate
fields. Then it copies into the Receive Timestamp and Transmit
Timestamp fields the data saved from the latest message received
from the destination peer and, finally, computes the Originate
Timestamp field.
The destination peer calculates the roundtrip delay and clock
offset relative to the source peer as follows. Let t1, t2 and t3
represent the contents of the Originate Timestamp, Receive
Timestamp and Transmit Timestamp fields and t4 the local time the
NTP message is received. Then the roundtrip delay d and clock
offset c is:
d = (t4 - t1) - (t3 - t2) and c = (t2 - t1 + t3 - t4)/2 .
The implicit assumption in the above is that the one-way delay is
statistically half the roundtrip delay and that the intrinsic
drift rates of both the client and server clocks are small and
close to the same value.
5.3. Network Considerations
The client/server peers have an opportunity to learn a good deal
about each other in the NTP message exchange. For instance, each
can learn about the characteristics of the other clocks and select
among them the most accurate to use as reference clock, compute
the estimated error and drift rate and use this information to
manage the dynamics of the subnetwork of clocks. An outline of a
suggested mechanism is as follows:
Included in the table of timestamps for each peer are state
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variables to indicate the precision, as well as the current
estimated delay, offset, error and drift rate of its local clock.
These variables are updated for each NTP message received from the
peer, after which the estimated error is periodically recomputed
on the basis of elapsed time and estimated drift rate.
Assuming symmetric mode, a polling interval is established for
each peer, depending upon its normal synchronization source,
precision and intrinsic accuracy, which might be determined in
advance or even as the result of observation. The delay and
clock-offset samples obtained can be filtered using
maximum-likelihood techniques and algorithms described in [12].
From time to time a local-clock correction is computed from the
offset data accumulated as above, perhaps using algorithms
described in [10] and [12]. The correction causes the local clock
to run slightly fast or slow to the corrected time or to jump
instantaneously to the correct time, depending on the magnitude of
the correction. See [5] and [11] for a discussion of local-clock
implementation models and synchronizing algorithms. Note that the
expectation here is that all network clocks are maintained by
these algorithms, so that manual intervention is not normally
required.
As a byproduct of the above operations an estimate of local-clock
error and drift rate can be computed. Note that the magnitude of
the error estimate must always be greater than that of the
selected reference clock by at least the inherent precision of the
local clock. It does not take a leap of imagination to see that
the estimated error, delay or precision, or some combination of
them, can be used as a metric for a simple min-hop-type routing
algorithm to organize the subnetwork so as to provide the most
accurate time to all peers and to provide automatic fallback to
alternate sources in case of failures.
A variety of network configurations can be included in the above
scenario. In the case of networks supporting a broadcast
function, for example, NTP messages can be broadcast from one or
more server hosts and picked up by client hosts sharing the same
cable. Since typical networks of this type have a very low
propagation delay, the roundtrip-delay calculation can be omitted
and the clients need not broadcast in return. Thus, the
requirement to save per-peer timestamps is removed, so that the
Receive Timestamp and Transmit Timestamp fields can be set to zero
and the local-clock offset becomes simply the difference between
the Originate Timestamp and the local time upon arrival. In the
case of long-delay satellite networks with broadcast capabilities,
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an accurate measure of roundtrip delay is usually available from
the channel-scheduling algorithm, so the per-peer timestamps again
can be avoided.
5.4. Leap Seconds
A standard mechanism to effect leap-second correction is not a
part of this specification. It is expected that the Leap
Indicator bits would be set by hand in the primary reference
clocks, then trickle down to all other clocks in the network,
which would execute the correction at the specified time and reset
the bits.
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6. References
1. Lindsay, W.C., and A.V. Kantak. Network Synchronization of
Random Signals. IEEE Trans. Comm. COM-28, 8 (August 1980),
1260-1266.
2. Mills, D.L. Time Synchronization in DCNET Hosts. DARPA Internet
Project Report IEN-173, COMSAT Laboratories, February 1981.
3. Mills, D.L. DCNET Internet Clock Service. DARPA Network Working
Group Report RFC-778, COMSAT Laboratories, April 1981.
4. Mills, D.L. Internet Delay Experiments. DARPA Network Working
Group Report RFC-889, M/A-COM Linkabit, December 1983.
5. Mills, D.L. DCN Local-Network Protocols. DARPA Network Working
Group Report RFC-891, M/A-COM Linkabit, December 1983.
6. Postel, J. Internet Control Message Protocol. DARPA Network
Working Group Report RFC-792, USC Information Sciences Institute,
September 1981.
7. Postel, J. Time Protocol. DARPA Network Working Group Report
RFC-868, USC Information Sciences Institute, May 1983.
8. Postel, J. Daytime Protocol. DARPA Network Working Group Report
RFC-867, USC Information Sciences Institute, May 1983.
9. Su, Z. A Specification of the Internet Protocol (IP) Timestamp
Option. DARPA Network Working Group Report RFC-781. SRI
International, May 1981.
10. Marzullo, K., and S. Owicki. Maintaining the Time in a
Distributed System. ACM Operating Systems Review 19, 3 (July
1985), 44-54.
11. Mills, D.L. Experiments in Network Clock Synchronization. DARPA
Network Working Group Report RFC-957, M/A-COM Linkabit, August
1985.
12. Mills, D.L. Algorithms for Synchronizing Network Clocks. DARPA
Network Working Group Report RFC-956, M/A-COM Linkabit, September
1985.
13. Postel, J. User Datagram Protocol. DARPA Network Working Group
Report RFC-768, USC Information Sciences Institute, August 1980.
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Appendix A. UDP Header Format
An NTP packet consists of the UDP header followed by the NTP data
portion. The format of the UDP header and the interpretation of its
fields are described in [13] and are not part of the NTP
specification. They are shown below for completeness.
UDP source port number. In the case of unsymmetric mode and a
client request this field is assigned by the client host, while
for a server reply it is copied from the Destination Port field of
the client request. In the case of symmetric mode, both the
Source Port and Destination Port fields are assigned the NTP
service-port number 123.
Destination Port
UDP destination port number. In the case of unsymmetric mode and a
client request this field is assigned the NTP service-port number
123, while for a server reply it is copied form the Source Port
field of the client request. In the case of symmetric mode, both
the Source Port and Destination Port fields are assigned the NTP
service-port number 123.
Length
Length of the request or reply, including UDP header, in octets.
Checksum
Standard UDP checksum.
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Appendix B. NTP Data Format
The format of the NTP data portion, which immediately follows the UDP
header, is shown below along with a description of its fields.
Code warning of impending leap-second to be inserted at the end of
the last day of the current month. Bits are coded as follows:
00 no warning
01 +1 second (following minute has 61 seconds)
10 -1 second (following minute has 59 seconds)
11 reserved for future use
Status
Code indicating status of local clock. Values are defined as
follows:
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0 clock operating correctly
1 carrier loss
2 synch loss
3 format error
4 interface (Type 1) or link (Type 2) failure
(additional codes reserved for future use)
Reference Clock Type
(Type)
Code identifying the type of reference clock. Values are defined
as follows:
0 unspecified
1 primary reference (e.g. radio clock)
2 secondary reference using an Internet host via NTP
3 secondary reference using some other host or protocol
4 eyeball-and-wristwatch
(additional codes reserved for future use)
Precision
Signed integer in the range +32 to -32 indicating the precision of
the local clock, in seconds to the nearest power of two.
Estimated Error
Fixed-point number indicating the estimated error of the local
clock at the time last set, in seconds with fraction point between
bits 15 and 16.
Estimated Drift Rate
Signed fixed-point number indicating the estimated drift rate of
the local clock, in dimensionless units with fraction point to the
left of the high-order bit.
Reference Clock
Identifier
Code identifying the particular reference clock. In the case of
type 1 (primary reference), this is a left-justified, zero-filled
ASCII string identifying the clock, for example:
WWVB WWVB radio clock (60 KHz)
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GOES GOES satellite clock (468 HMz)
WWV WWV radio clock (2.5/5/10/15/20 MHz)
(and others as necessary)
In the case of type 2 (secondary reference) this is the 32-bit
Internet address of the reference host. In other cases this field
is reserved for future use and should be set to zero.
Reference Timestamp
Local time at which the local clock was last set or corrected.
Originate Timestamp
Local time at which the request departed the client host for the
service host.
Receive Timestamp
Local time at which the request arrived at the service host.
Transmit Timestamp
Local time at which the reply departed the service host for the
client host.
