RFC1361 - Simple Network Time Protocol (SNTP)

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　　Network Working Group D. Mills
Request for Comments: 1361 University of Delaware
August 1992

Simple Network Time Protocol (SNTP)

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.

Abstract

This memorandum describes the Simple Network Time Protocol (SNTP),
which is an adaptation of the Network Time Protocol (NTP) used to
synchronize computer clocks in the Internet. SNTP can be used when
the ultimate performance of the full NTP implementation described in
RFC-1305 is not needed or justified. It involves no change to the
current or previous NTP specification versions or known
implementations, but rather a clarification of certain design
features of NTP which allow operation in a simple, stateless RPC mode
with accuracy and reliability expectations similar to the UDP/TIME
protocol described in RFC-868.

This memorandum does not obsolete or update any RFC. A working
knowledge of RFC-1305 is not required for an implementation of SNTP.

1. Introduction

The Network Time Protocol (NTP) specified in RFC-1305 [MIL92] is used
to synchronize computer clocks in the global Internet. It provides
comprehensive mechanisms to access national time and frequency
dissemination services, organize the time-synchronization subnet and
adjust the local clock in each participating subnet peer. In most
places of the Internet of today, NTP provides accuracies of 1-50 ms,
depending on the jitter characteristics of the synchronization source
and network paths.

RFC-1305 specifies the NTP protocol machine in terms of events,
states, transition functions and actions and, in addition, optional
algorithms to improve the timekeeping quality and mitigate among
several, possibly faulty, synchronization sources. To achieve
accuracies in the low milliseconds over paths spanning major portions
of the Internet of today, these intricate algorithms, or their
functional equivalents, are necessary. However, in many cases
accuracies of this order are not required and something less, perhaps

in the order of one second, is sufficient. In such cases simpler
protocols such as the Time Protocol [POS83], have been used for this
purpose. These protocols usually involve a remote-procedure call
(RPC) exchange where the client requests the time of day and the
server returns it in seconds past some known reference epoch.

NTP is designed for use by clients and servers with a wide range of
capabilities and over a wide range of network delays and jitter
characteristics. Most members of the Internet NTP synchronization
subnet of today use software packages including the full suite of NTP
options and algorithms, which are relatively complex, real-time
applications. While the software has been ported to a wide variety of
hardware platforms ranging from supercomputers to personal computers,
its sheer size and complexity is not appropriate for many
applications. Accordingly, it is useful to explore alternative access
strategies using far simpler software appropriate for accuracy
expectations in the order of a second.

This memorandum describes the Simple Network Time Protocol (SNTP),
which is a simplified access strategy for servers and clients using
NTP as now specified and deployed in the Internet. There are no
changes to the protocol or implementations now running or likely to
be implemented in the near future. The access paradigm is identical
to the UDP/Time Protocol and, in fact, it should be easily possible
to adapt a UDP/Time client implementation, say for a personal
computer, to operate using SNTP. Moreover, SNTP is also designed to
operate in a dedicated server configuration including an integrated
radio clock. With careful design and control of the various latencies
in the system, which is practical in a dedicated design, it is
possible to deliver time accurate to the order of microseconds.

It is strongly recommended that SNTP be used only at the extremities
of the synchronization subnet. SNTP clients should operate only at
the leaves (highest stratum) of the subnet and in configurations
where no SNTP client is dependent on another SNTP client for
synchronization. SNTP servers should operate only at the root
(stratum 1) of the subnet and then only in configurations where no
other source of synchronization other than a reliable radio clock is
available. The full degree of reliability ordinarily expected of
primary servers is possible only using the redundant sources, diverse
subnet paths and crafted algorithms of a full NTP implementation.
This extends to the primary source of synchronization itself in the
form of multiple radio clocks and backup paths to other primary
servers should the radio clock fail or become faulty. Therefore, the
use of SNTP rather than NTP in primary servers should be carefully
considered.

2. NTP Timestamp Format

SNTP uses the standard NTP timestamp format described in RFC-1305 and
previous versions of that document. In conformance with standard
Internet practice, NTP data are specified as integer or fixed-point
quantities, with bits numbered in big-endian fashion from zero
starting at the left, or high-order, position. Unless specified
otherwise, all quantities are unsigned and may occupy the full field
width with an implied zero preceding bit zero.

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 0h 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.

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 or SNTP 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
a 200-picosecond interval, henceforth ignored, every 136 years when
the 64-bit field will be zero, which by convention is interpreted as
an invalid timestamp.

3. NTP Message Format

Both NTP and SNTP are clients of the User Datagram Protocol (UDP)
[POS83], which itself is a client of the Internet Protocol (IP)
[DAR81]. The structure of the IP and UDP headers is described in the
relevant specification documents and will not be described further in
this memorandum. Following is a description of the SNTP message
format, which follows the IP and UDP headers. The SNTP message format
is identical to the NTP format described in RFC-1305, with the
exception that some of the data fields are "canned," that is,
initialized to prespecified values. The format of the NTP message
data area, which immediately follows the UDP header, is shown below.

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|LI | VN |Mode | Stratum | Poll | Precision |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Root Dispersion |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reference Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Reference Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Originate Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Receive Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Transmit Timestamp (64) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| Authenticator (optional) (96) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

As described in the next section, in SNTP most of these fields are
initialized with prespecified data. For completeness, the function of
each field is briefly summarized below.

Leap Indicator (LI): This is a two-bit code warning of an impending
leap second to be inserted/deleted in the last minute of the current
day, with bit 0 and bit 1, respectively, coded as follows:

LI Value Meaning
-------------------------------------------------------
00 0 no warning
01 1 last minute has 61 seconds
10 2 last minute has 59 seconds)
11 3 alarm condition (clock not synchronized)

Version Number (VN): This is a three-bit integer indicating the NTP
version number, currently 3.

Mode: This is a three-bit integer indicating the mode, with values
defined as follows:

Mode Meaning
------------------------------------
0 reserved
1 symmetric active
2 symmetric passive
3 client
4 server
5 broadcast
6 reserved for NTP control message
7 reserved for private use

The use of this field will be discussed in the next section.

Stratum: This is a eight-bit integer indicating the stratum level of
the local clock, with values defined as follows:

Stratum Meaning
----------------------------------------------
0 unspecified or unavailable
1 primary reference (e.g., radio clock)
2-15 secondary reference (via NTP or SNTP)
16-255 reserved

Poll Interval: This is an eight-bit signed integer indicating the
maximum interval between successive messages, in seconds to the
nearest power of two. The values that normally appear in this field
range from 6 to 10, inclusive.

Precision: This is an eight-bit signed integer indicating the
precision of the local clock, in seconds to the nearest power of two.
The values that normally appear in this field range from -6 for
mains-frequency clocks to -18 for microsecond clocks found in some
workstations.

Root Delay: This is a 32-bit signed fixed-point number indicating the
total roundtrip delay to the primary reference source, in seconds
with fraction point between bits 15 and 16. Note that this variable
can take on both positive and negative values, depending on the
relative time and frequency errors. The values that normally appear
in this field range from negative values of a few milliseconds to
positive values of several hundred milliseconds.

Root Dispersion: This is a 32-bit unsigned fixed-point number
indicating the maximum error relative to the primary reference
source, in seconds with fraction point between bits 15 and 16. The
values that normally appear in this field range from zero to several
hundred milliseconds.

Reference Clock Identifier: This is a 32-bit code identifying the
particular reference clock. In the case of stratum 0 (unspecified) or
stratum 1 (primary reference), this is a four-octet, left-justified,
zero-padded ASCII string. While not enumerated as part of the NTP
specification, the following are representative ASCII identifiers:

Stratum Code Meaning
------------------------------------------------------------
0 ascii generic time service other than NTP, such as ACTS
(Automated Computer Time Service), TIME (UDP/Time
Protocol), TSP (TSP Unix time protocol), DTSS
(Digital Time Synchronization Service), etc.
1 ATOM calibrated atomic clock
1 VLF VLF radio (OMEGA, etc.)
1 callsign Generic radio
1 LORC LORAN-C radionavigation system
1 GOES Geostationary Operational Environmental Satellite
1 GPS Global Positioning Service
2 address secondary reference (four-octet Internet address of
the NTP server)

Reference Timestamp: This is the local time at which the local clock
was last set or corrected, in 64-bit timestamp format.

Originate Timestamp: This is the local time at which the request
departed the client for the server, in 64-bit timestamp format.

Receive Timestamp: This is the local time at which the request
arrived at the server, in 64-bit timestamp format.

Transmit Timestamp: This is the local time at which the reply
departed the server for the client, in 64-bit timestamp format.

Authenticator (optional): When the NTP authentication mechanism is
implemented, this contains the authenticator information defined in
Appendix C of RFC-1305. In SNTP this field is ignored for incoming
messages and is not generated for outgoing messages.

4. SNTP Client Operations

The model for an SNTP client operating with either an NTP or SNTP
server is a RPC client with no persistent state. The client
initializes the SNTP message header, sends the message to the server
and strips the time of day from the reply. For this purpose all of
the message-header fields shown above are set to zero, except the
first octet. In this octet the Leap Indicator is set to zero (no
warning) and the Mode to 3 (client). The Version Number must agree
with the software version of the NTP or SNTP server; however, NTP
Version 3 (RFC-1305) servers will also accept Version 2 (RFC-1119)
and Version 1 (RFC-1059) messages, while NTP Version 2 servers will
also accept NTP Version 1 messages. Version 0 (original NTP described
in RFC-959) messages are no longer supported. Since there are NTP
servers of all three versions operating in the Internet of today, it
is recommended that the Version Number field be set to one.

The server reply includes all the fields described above; however, in
SNTP only the Transmit Timestamp has explicit meaning. The integer
part of this field contains the server time of day in the same format
as the Time Protocol. While the fraction part of this field will
usually be valid, the accuracy achieved with the SNTP mode of access
probably does not justify its use.

The following table is a summary of the SNTP client operations. There
are three recommended error checks shown in the table. In all NTP
versions, if the Leap Indicator field is 3 or the Transmit Timestamp
is zero (unsynchronized), the server has never synchronized or not
synchronized to a valid timing source within the last 24 hours. If
the Stratum field is 0 (unspecified or unavailable), the server has
never synchronized, has lost reachability with all timing sources or
is synchronized by some protocol other than NTP. Whether to believe
the transmit timestamp or not in this case is at the discretion of
the client implementation.

Field Name Request Reply
-------------------------------------------------------------
Leap Indicator (LI) 0 if 3 (unsynchronized),
disregard
Version Number (VN) (see text) ignore
Mode 3 (client) ignore
Stratum 0 if 0 (unspecified),
disregard
Poll 0 ignore
Precision 0 ignore
Root Delay 0 ignore
Root Dispersion 0 ignore
Reference Identifier 0 ignore
Reference Timestamp 0 ignore
Originate Timestamp 0 ignore
Receive Timestamp 0 ignore
Transmit Timestamp 0 time of day (seconds only);
if 0 (unsynchronized),
disregard
Authenticator (not used) ignore

5. SNTP Server Operations

The model for an SNTP server operating with either an NTP or SNTP
client is an RPC server with no persistent state. The SNTP server
ignores all header fields except the first octet, modifies certain
fields and returns the message to the sender. Since an SNTP server
ordinarily does not implement the full set of NTP algorithms intended
to support the highest quality service, it is recommended that an
SNTP server be operated only in conjunction with a source of outside
synchronization, such as a radio clock. In this case the server
always operates at stratum 1.

The first octet is interpreted as follows. The Leap Indicator and
Version Number fields are ignored. Optionally, messages with version
numbers other than 1, 2, or 3 can be discarded. For primary servers
connected to a functioning radio clock, the Leap Indicator field is
set to zero and the Stratum field is set to one in the reply.
otherwise, these fields are set to 3 and zero, respectively. In any
case the Version Number and Poll fields are copied intact to the
reply message header. If The Mode field is set to 3 (client), it is
changed to 4 (server) in the reply; otherwise, this field is set to 2
(symmetric passive).

The Stratum field is set to reflect the maximum reading error of the
local clock. For all practical cases it is computed as the negative
of the number of significant bits to the right of the decimal point
in the NTP timestamp format. The Root Delay and Root Dispersion

fields are set to zero for a primary server; optionally, the Root
Dispersion can be set to a value corresponding to the expected
(constant) maximum expected error of the primary reference source.
The Reference Identifier is set to designate the primary reference
source, as indicated in the table above. If this information is
unspecified or unavailable, the field is set to zero.

The timestamp fields are set as follows. The Reference Timestamp,
Receive Timestamp and Transmit Timestamp fields are set to the time
of day at the server. The Originate Timestamp field is copied
unchanged from the request. The following table summarizes these
actions.

Field Name Request Reply
----------------------------------------------------------
Leap Indicator (LI) ignore 0 (normal), 3
(unsynchronized)
Version Number (VN) ignore copied from request
Mode (see text) (see text)
Stratum ignore server stratum (1)
Poll ignore copied from request
Precision ignore server precision
Root Delay ignore 0
Root Dispersion ignore 0 (see text)
Reference Identifier ignore source identifier or 0
Reference Timestamp ignore time of day or 0
Originate Timestamp ignore copied from request
Receive Timestamp ignore time of day or 0
Transmit Timestamp ignore time of day or 0
Authenticator ignore (not used)

6. References

[DAR81] Postel, J., "Internet Protocol - DARPA Internet Program
Protocol Specification", RFC791, DARPA, September 1981.

[MIL92] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation and Analysis", RFC1305, University of Delaware,
March 1992.

[POS80] Postel, J., "User Datagram Protocol", RFC768,
USC/Information Sciences Institute, August 1980.

[POS83] Postel, J., and K. Harrenstien, "Time Protocol", RFC868,
USC/Information Sciences Institute, SRI, May 1983.

Security Considerations

Security issues are not discussed in this memo.

Author's Address

David L. Mills
Electrical Engineering Department
University of Delaware
Newark, DE 19716

Phone: (302) 831-8247

EMail: mills@udel.edu

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