Network Working Group D. Mills
Request for Comments: 4330 University of Delaware
Obsoletes: 2030, 1769 January 2006
Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright (C) The Internet Society (2006).
This memorandum describes the Simple Network Time Protocol Version 4
(SNTPv4), which is a subset of the Network Time Protocol (NTP) used
to synchronize computer clocks in the Internet. SNTPv4 can be used
when the ultimate performance of a full NTP implementation based on
RFC 1305 is neither needed nor justified. When operating with
current and previous NTP and SNTP versions, SNTPv4 requires no
changes to the specifications or known implementations, but rather
clarifies certain design features that allow operation in a simple,
stateless remote-procedure call (RPC) mode with accuracy and
reliability expectations similar to the UDP/TIME protocol described
in RFC 868.
This memorandum obsoletes RFC 1769, which describes SNTP Version 3
(SNTPv3), and RFC 2030, which describes SNTPv4. Its purpose is to
correct certain inconsistencies in the previous documents and to
clarify header formats and protocol operations for NTPv3 (IPv4) and
SNTPv4 (IPv4, IPv6, and OSI), which are also used for SNTP. A
further purpose is to provide guidance for home and business client
implementations for routers and other consumer devices to protect the
server population from abuse. A working knowledge of the NTPv3
specification, RFC 1305, is not required for an implementation of
Table of Contents
1. Introduction ....................................................21.1. Specification of Requirements ..............................52. Operating Modes and Addressing ..................................53. NTP Timestamp Format ............................................64. Message Format ..................................................85. SNTP Client Operations .........................................136. SNTP Server Operations .........................................167. Configuration and Management ...................................198. The Kiss-o'-Death Packet .......................................209. On Being a Good Network Citizen ................................2110. Best Practices ................................................2111. Security Considerations .......................................2412. Acknowledgements ..............................................2413. Contributors ..................................................2414. Informative References ........................................251. Introduction
The Network Time Protocol Version 3 (NTPv3), specified in RFC 1305
[MIL92], is widely used to synchronize computer clocks in the global
Internet. It provides comprehensive mechanisms to access national
time and frequency dissemination services, organize the NTP subnet of
servers and clients, and adjust the system clock in each participant.
In most places of the Internet of today, NTP provides accuracies of
1-50 ms, depending on the 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 engineered algorithms
to improve the timekeeping quality and to mitigate several
synchronization sources, some of which may be faulty. To achieve
accuracies in the low milliseconds over paths spanning major portions
of the Internet, these intricate algorithms, or their functional
equivalents, are necessary. In many applications, accuracies on the
order of significant fractions of a second are acceptable. In simple
home router applications, accuracies of up to a minute may suffice.
In such cases, simpler protocols, such as the Time Protocol specified
in RFC 868 [POS83], have been used for this purpose. These protocols
involve an RPC exchange where the client requests the time of day and
the server returns it in seconds past a 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 jitter and clock
frequency wander characteristics. Many users of NTP in the Internet
of today use a software distribution available from www.ntp.org. The
distribution, which includes the full suite of NTP options,
mitigation algorithms, and security schemes, is a relatively complex,
real-time application. Although the software has been ported to a
wide variety of hardware platforms ranging from personal computers to
supercomputers, its sheer size and complexity is not appropriate for
many applications. Accordingly, it is useful to explore alternative
strategies using simpler software appropriate for less stringent
This memo describes the Simple Network Time Protocol Version 4
(SNTPv4), which is a simplified access paradigm for servers and
clients using current and previous versions of NTP and SNTP. The
access paradigm is identical to the UDP/TIME Protocol, and, in fact,
it should be easy 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 on the order of
The only significant protocol change in SNTPv4 from previous SNTP
versions is a modified header interpretation to accommodate Internet
Protocol Version 6 (IPv6) (RFC 2460) and OSI (RFC 1629) addressing.
However, SNTPv4 includes certain optional extensions to the basic NTP
Version 3 (NTPv3) model, including a manycast mode and a public-key-
based authentication scheme designed specifically for broadcast and
manycast applications. Although the manycast mode is described in
this memo, the authentication scheme is described in another RFC to
be submitted later. Until such time that a definitive NTPv4
specification is published, the manycast and authentication features
should be considered provisional. In addition, this memo introduces
the kiss-o'-death message, which can be used by servers to suppress
client requests as circumstances require.
When operating with current and previous versions of NTP and SNTP,
SNTPv4 requires no changes to the protocol or implementations now
running or likely to be implemented specifically for future NTP or
SNTP versions. The NTP and SNTP packet formats are the same, and the
arithmetic operations to calculate the client time, clock offset, and
roundtrip delay are the same. To an NTP or SNTP server, NTP and SNTP
clients are indistinguishable; to an NTP or SNTP client, NTP and SNTP
servers are indistinguishable. Like NTP servers operating in non-
symmetric modes, SNTP servers are stateless and can support large
numbers of clients; however, unlike most NTP clients, SNTP clients
normally operate with only a single server at a time.
The full degree of reliability ordinarily expected of NTP servers is
possible only using redundant sources, diverse paths, and the crafted
algorithms of a full NTP implementation. It is strongly recommended
that SNTP clients 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
NTP or 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 or
telephone modem is available.
An important provision in this memo is the interpretation of certain
NTP header fields that provide for IPv6 [DEE98] and OSI [COL94]
addressing. The only significant difference between the NTP and
SNTPv4 header formats is the four-octet Reference Identifier field,
which is used primarily to detect and avoid synchronization loops.
In all NTP and SNTP versions providing IPv4 addressing, primary
servers use a four-character ASCII reference clock identifier in this
field, whereas secondary servers use the 32-bit IPv4 address of the
synchronization source. In SNTPv4 providing IPv6 and OSI addressing,
primary servers use the same clock identifier, but secondary servers
use the first 32 bits of the MD5 hash of the IPv6 or NSAP address of
the synchronization source. A further use of this field is when the
server sends a kiss-o'-death message, documented later in this memo.
NTP Version 4 (NTPv4), now in deployment, but not yet the subject
of a standards document, uses the same Reference Identifier field
In the case of OSI, the Connectionless Transport Service (CLTS) is
used as in [ISO86]. Each SNTP packet is transmitted as the TS-
Userdata parameter of a T-UNITDATA Request primitive. Alternately,
the header can be encapsulated in a Transport Protocol Data Unit
(TPDU), which itself is transported using UDP, as described in RFC
1240 [DOB91]. It is not advised that NTP be operated at the upper
layers of the OSI stack, such as might be inferred from RFC 1698
[FUR94], as this could seriously degrade accuracy. With the header
formats defined in this memo, it is in principle possible to
interwork between servers and clients of one protocol family and
another, although the practical difficulties may make this
In the following, indented paragraphs such as this one contain
information not required by the formal protocol specification, but
considered good practice in protocol implementations.
This memo is organized as follows. Section 2 describes how the
protocol works, the various modes, and how IP addresses and UDP ports
are used. Section 3 describes the NTP timestamp format, and Section
4 the NTP message format. Section 5 summarizes SNTP client
operations, and Section 6 summarizes SNTP server operations. Section
7 summarizes operation and management issues. Section 8 describes
the kiss-o'-death message, newly minted with functions similar to the
ICMP Source Quench and ICMP Destination Unreachable messages.
Section 9 summarizes design issues important for good network
citizenry and presents an example algorithm designed to give good
reliability while minimizing network and server resource demands.
1.1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [BRA97].
2. Operating Modes and Addressing
Unless excepted in context, a reference to broadcast address means
IPv4 broadcast address, IPv4 multicast group address, or IPv6 address
of appropriate scope. Further information on the broadcast/multicast
model is in RFC 1112 [DEE89]. Details of address format, scoping
rules, etc., are beyond the scope of this memo. SNTPv4 can operate
with either unicast (point to point), broadcast (point to
multipoint), or manycast (multipoint to point) addressing modes. A
unicast client sends a request to a designated server at its unicast
address and expects a reply from which it can determine the time and,
optionally, the roundtrip delay and clock offset relative to the
server. A broadcast server periodically sends an unsolicited message
to a designated broadcast address. A broadcast client listens on
this address and ordinarily sends no requests.
Manycast is an extension of the anycast paradigm described in RFC
1546 [PAR93]. It is designed for use with a set of cooperating
servers whose addresses are not known beforehand. The manycast
client sends an ordinary NTP client request to a designated broadcast
address. One or more manycast servers listen on that address. Upon
receiving a request, a manycast server sends an ordinary NTP server
reply to the client. The client then mobilizes an association for
each server found and continues operation with all of them.
Subsequently, the NTP mitigation algorithms operate to cast out all
except the best three.
Broadcast servers should respond to client unicast requests, as
well as send unsolicited broadcast messages. Broadcast clients
may send unicast requests in order to measure the network
propagation delay between the server and client and then continue
operation in listen-only mode. However, broadcast servers may
choose not to respond to unicast requests, so unicast clients
should be prepared to abandon the measurement and assume a default
value for the delay.
The client and server addresses are assigned following the usual
IPv4, IPv6 or OSI conventions. For NTP multicast, the IANA has
reserved the IPv4 group address 18.104.22.168 and the IPv6 address ending
:101 with appropriate scope. The NTP broadcast address for OSI has
yet to be determined. Notwithstanding the IANA reserved addresses,
other multicast addresses can be used that do not conflict with
others assigned in scope. The scoping, routing, and group membership
procedures are determined by considerations beyond the scope of this
It is important to adjust the time-to-live (TTL) field in the IP
header of multicast messages to a reasonable value in order to
limit the network resources used by this (and any other) multicast
service. Only multicast clients in scope will receive multicast
server messages. Only cooperating manycast servers in scope will
reply to a client request. The engineering principles that
determine the proper values to be used are beyond the scope of
In the case of SNTP as specified herein, there is a very real
vulnerability that SNTP broadcast clients can be disrupted by
misbehaving or hostile SNTP or NTP broadcast servers elsewhere in
the Internet. It is strongly recommended that access controls
and/or cryptographic authentication means be provided for
additional security in such cases.
It is intended that IP broadcast addresses will be used primarily
in IP subnets and LAN segments including a fully functional NTP
server with a number of dependent SNTP broadcast clients on the
same subnet, and that IP multicast group addresses will be used
only in cases where the TTL is engineered specifically for each
service domain. However, these uses are not integral to the SNTP
3. 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 0 starting
at the left or most significant end. Unless specified otherwise, all
quantities are unsigned and may occupy the full field width with an
implied 0 preceding bit 0.
Because 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. In the fraction part, the non-significant low-order
bits are not specified and are ordinarily set to 0.
It is advisable to fill the non-significant low-order bits of the
timestamp with a random, unbiased bitstring, both to avoid
systematic roundoff errors and to provide loop detection and
replay detection (see below). It is important that the bitstring
be unpredictable by an intruder. One way of doing this is to
generate a random 128-bit bitstring at startup. After that, each
time the system clock is read, the string consisting of the
timestamp and bitstring is hashed with the MD5 algorithm, then the
non-significant bits of the timestamp are copied from the result.
The NTP format allows convenient multiple-precision arithmetic and
conversion to UDP/TIME message (seconds), but does complicate the
conversion to ICMP Timestamp message (milliseconds) and Unix time
values (seconds and microseconds or seconds and nanoseconds). The
maximum number that can be represented is 4,294,967,295 seconds with
a precision of about 232 picoseconds, which should be adequate for
even the most exotic requirements.
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
| Seconds |
| Seconds Fraction (0-padded) |
Note that since some time in 1968 (second 2,147,483,648), 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 (second 4,294,967,296).
There will exist a 232-picosecond interval, henceforth ignored, every
136 years when the 64-bit field will be 0, which by convention is
interpreted as an invalid or unavailable timestamp.
As the NTP timestamp format has been in use for over 20 years, it
is possible that it will be in use 32 years from now, when the
seconds field overflows. As it is probably inappropriate to
archive NTP timestamps before bit 0 was set in 1968, a convenient
way to extend the useful life of NTP timestamps is the following
convention: If bit 0 is set, the UTC time is in the range 1968-
2036, and UTC time is reckoned from 0h 0m 0s UTC on 1 January
1900. If bit 0 is not set, the time is in the range 2036-2104 and
UTC time is reckoned from 6h 28m 16s UTC on 7 February 2036. Note
that when calculating the correspondence, 2000 is a leap year, and
leap seconds are not included in the reckoning.
The arithmetic calculations used by NTP to determine the clock
offset and roundtrip delay require the client time to be within 34
years of the server time before the client is launched. As the
time since the Unix base 1970 is now more than 34 years, means
must be available to initialize the clock at a date closer to the
present, either with a time-of-year (TOY) chip or from firmware.
4. Message Format
Both NTP and SNTP are clients of the User Datagram Protocol (UDP)
specified in RFC 768 [POS80]. The structures of the IP and UDP
headers are described in the cited specification documents and will
not be detailed further here. The UDP port number assigned by the
IANA to NTP is 123. The SNTP client should use this value in the UDP
Destination Port field for client request messages. The Source Port
field of these messages can be any nonzero value chosen for
identification or multiplexing purposes. The server interchanges
these fields for the corresponding reply messages.
This differs from the RFC 2030 specifications, which required both
the source and destination ports to be 123. The intent of this
change is to allow the identification of particular client
implementations (which are now allowed to use unreserved port
numbers, including ones of their choosing) and to attain
compatibility with Network Address Port Translation (NAPT)
described in RFC 2663 [SRI99] and RFC 3022 [SRI01].
Figure 1 is a description of the NTP and SNTP message format, which
follows the IP and UDP headers in the message. This format is
identical to the NTP message format described in RFC 1305, with the
exception of the Reference Identifier field described below. For
SNTP client messages, most of these fields are zero or initialized
with pre-specified data. For completeness, the function of each
field is briefly summarized below.
0 no warning
1 last minute has 61 seconds
2 last minute has 59 seconds
3 alarm condition (clock not synchronized)
On startup, servers set this field to 3 (clock not synchronized), and
set this field to some other value when synchronized to the primary
reference clock. Once set to a value other than 3, the field is
never set to that value again, even if all synchronization sources
become unreachable or defective.
Version Number (VN): This is a three-bit integer indicating the
NTP/SNTP version number, currently 4. If necessary to distinguish
between IPv4, IPv6, and OSI, the encapsulating context must be
Mode: This is a three-bit number indicating the protocol mode. The
values are defined as follows:
1 symmetric active
2 symmetric passive
6 reserved for NTP control message
7 reserved for private use
In unicast and manycast modes, the client sets this field to 3
(client) in the request, and the server sets it to 4 (server) in the
reply. In broadcast mode, the server sets this field to 5
(broadcast). The other modes are not used by SNTP servers and
Stratum: This is an eight-bit unsigned integer indicating the
stratum. This field is significant only in SNTP server messages,
where the values are defined as follows:
0 kiss-o'-death message (see below)
1 primary reference (e.g., synchronized by radio clock)
2-15 secondary reference (synchronized by NTP or SNTP)
Poll Interval: This is an eight-bit unsigned integer used as an
exponent of two, where the resulting value is the maximum interval
between successive messages in seconds. This field is significant
only in SNTP server messages, where the values range from 4 (16 s) to
17 (131,072 s -- about 36 h).
Precision: This is an eight-bit signed integer used as an exponent of
two, where the resulting value is the precision of the system clock
in seconds. This field is significant only in server messages, where
the values range from -6 for mains-frequency clocks to -20 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 the 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 offsets. This field is significant
only in server messages, where the values range from negative values
of a few milliseconds to positive values of several hundred
Code External Reference Source
LOCL uncalibrated local clock
CESM calibrated Cesium clock
RBDM calibrated Rubidium clock
PPS calibrated quartz clock or other pulse-per-second
IRIG Inter-Range Instrumentation Group
ACTS NIST telephone modem service
USNO USNO telephone modem service
PTB PTB (Germany) telephone modem service
TDF Allouis (France) Radio 164 kHz
DCF Mainflingen (Germany) Radio 77.5 kHz
MSF Rugby (UK) Radio 60 kHz
WWV Ft. Collins (US) Radio 2.5, 5, 10, 15, 20 MHz
WWVB Boulder (US) Radio 60 kHz
WWVH Kauai Hawaii (US) Radio 2.5, 5, 10, 15 MHz
CHU Ottawa (Canada) Radio 3330, 7335, 14670 kHz
LORC LORAN-C radionavigation system
OMEG OMEGA radionavigation system
GPS Global Positioning Service
Figure 2. Reference Identifier Codes
Root Dispersion: This is a 32-bit unsigned fixed-point number
indicating the maximum error due to the clock frequency tolerance, in
seconds with the fraction point between bits 15 and 16. This field
is significant only in server messages, where the values range from
zero to several hundred milliseconds.
Reference Identifier: This is a 32-bit bitstring identifying the
particular reference source. This field is significant only in
server messages, where for stratum 0 (kiss-o'-death message) and 1
(primary server), the value is a four-character ASCII string, left
justified and zero padded to 32 bits. For IPv4 secondary servers,
the value is the 32-bit IPv4 address of the synchronization source.
For IPv6 and OSI secondary servers, the value is the first 32 bits of
the MD5 hash of the IPv6 or NSAP address of the synchronization
Primary (stratum 1) servers set this field to a code identifying the
external reference source according to Figure 2. If the external
reference is one of those listed, the associated code should be used.
Codes for sources not listed can be contrived, as appropriate.
In previous NTP and SNTP secondary servers and clients, this field
was often used to walk-back the synchronization subnet to the root
(primary server) for management purposes. In SNTPv4 with IPv6 or
OSI, this feature is not available, because the addresses are
longer than 32 bits, and only a hash is available. However, a
walk-back can be accomplished using the NTP control message and
the reference identifier field described in RFC 1305.
Reference Timestamp: This field is the time the system clock was last
set or corrected, in 64-bit timestamp format.
Originate Timestamp: This is the time at which the request departed
the client for the server, in 64-bit timestamp format.
Receive Timestamp: This is the time at which the request arrived at
the server or the reply arrived at the client, in 64-bit timestamp
Transmit Timestamp: This is the time at which the request departed
the client or the reply departed the server, in 64-bit timestamp
Authenticator (optional): When the NTP authentication scheme is
implemented, the Key Identifier and Message Digest fields contain the
message authentication code (MAC) information defined in Appendix C
of RFC 1305.
5. SNTP Client Operations
An SNTP client can operate in unicast, broadcast, or manycast modes.
In unicast mode, the client sends a request (NTP mode 3) to a
designated unicast server and expects a reply (NTP mode 4) from that
server. In broadcast client mode, it sends no request and waits for
a broadcast (NTP mode 5) from one or more broadcast servers. In
manycast mode, the client sends a request (NTP mode 3) to a
designated broadcast address and expects a reply (NTP mode 4) from
one or more manycast servers. The client uses the first reply
received to establish the particular server for subsequent unicast
operations. Later replies from this server (duplicates) or any other
server are ignored. Other than the selection of address in the
request, the operations of manycast and unicast clients are
Client requests are normally sent at intervals depending on the
frequency tolerance of the client clock and the required accuracy.
However, under no conditions should requests be sent at less than
one minute intervals. Further discussion on this point is in
A unicast or manycast client initializes the NTP message header,
sends the request to the server, and strips the time of day from the
Transmit Timestamp field of the reply. For this purpose, all the NTP
header fields shown above are set to 0, except the Mode, VN, and
optional Transmit Timestamp fields.
NTP and SNTP clients set the mode field to 3 (client) for unicast and
manycast requests. They set the VN field to any version number that
is supported by the server, selected by configuration or discovery,
and that can interoperate with all previous version NTP and SNTP
servers. Servers reply with the same version as the request, so the
VN field of the request also specifies the VN field of the reply. A
prudent SNTP client can specify the earliest acceptable version on
the expectation that any server of that or a later version will
respond. NTP Version 3 (RFC 1305) and Version 2 (RFC 1119) servers
accept all previous versions, including Version 1 (RFC 1059). Note
that Version 0 (RFC 959) is no longer supported by current and future
NTP and SNTP servers.
Although setting the Transmit Timestamp field in the request to the
time of day according to the client clock in NTP timestamp format is
not necessary in a conforming client implementation, it is highly
recommended in unicast and manycast modes. This allows a simple
calculation to determine the propagation delay between the server and
client and to align the system clock generally within a few tens of
milliseconds relative to the server. In addition, this provides a
simple method for verifying that the server reply is in fact a
legitimate response to the specific client request and thereby for
avoiding replays. In broadcast mode, the client has no information
to calculate the propagation delay or to determine the validity of
the server, unless one of the NTP authentication schemes is used.
To calculate the roundtrip delay d and system clock offset t relative
to the server, the client sets the Transmit Timestamp field in the
request to the time of day according to the client clock in NTP
timestamp format. For this purpose, the clock need not be
synchronized. The server copies this field to the Originate
Timestamp in the reply and sets the Receive Timestamp and Transmit
Timestamp fields to the time of day according to the server clock in
NTP timestamp format.
When the server reply is received, the client determines a
Destination Timestamp variable as the time of arrival according to
its clock in NTP timestamp format. The following table summarizes
the four timestamps.
Timestamp Name ID When Generated
Originate Timestamp T1 time request sent by client
Receive Timestamp T2 time request received by server
Transmit Timestamp T3 time reply sent by server
Destination Timestamp T4 time reply received by client
The roundtrip delay d and system clock offset t are defined as:
d = (T4 - T1) - (T3 - T2) t = ((T2 - T1) + (T3 - T4)) / 2.
Note that in general both delay and offset are signed quantities and
can be less than zero; however, a delay less than zero is possible
only in symmetric modes, which SNTP clients are forbidden to use.
The following table summarizes the required SNTP client operations in
unicast, manycast, and broadcast modes. The recommended error checks
are shown in the Reply and Broadcast columns in the table. The
message should be considered valid only if all the fields shown
contain values in the respective ranges. Whether to believe the
message if one or more of the fields marked "ignore" contain invalid
values is at the discretion of the implementation.
Field Name Unicast/Manycast Broadcast
LI 0 0-3 0-3
VN 1-4 copied from 1-4
Mode 3 4 5
Stratum 0 0-15 0-15
Poll 0 ignore ignore
Precision 0 ignore ignore
Root Delay 0 ignore ignore
Root Dispersion 0 ignore ignore
Reference Identifier 0 ignore ignore
Reference Timestamp 0 ignore ignore
Originate Timestamp 0 (see text) ignore
Receive Timestamp 0 (see text) ignore
Transmit Timestamp (see text) nonzero nonzero
Authenticator optional optional optional
Although not required in a conforming SNTP client implementation, it
is wise to consider a suite of sanity checks designed to avoid
various kinds of abuse that might happen as the result of server
implementation errors or malicious attack. Following is a list of
1. When the IP source and destination addresses are available for
the client request, they should match the interchanged addresses
in the server reply.
2. When the UDP source and destination ports are available for the
client request, they should match the interchanged ports in the
3. The Originate Timestamp in the server reply should match the
Transmit Timestamp used in the client request.
4. The server reply should be discarded if any of the LI, Stratum,
or Transmit Timestamp fields is 0 or the Mode field is not 4
(unicast) or 5 (broadcast).
5. A truly paranoid client can check that the Root Delay and Root
Dispersion fields are each greater than or equal to 0 and less
than infinity, where infinity is currently a cozy number like one
second. This check avoids using a server whose synchronization
source has expired for a very long time.
6. SNTP Server Operations
A SNTP server operating with either an NTP or SNTP client of the same
or previous versions retains no persistent state. Because an SNTP
server ordinarily does not implement the full suite of grooming and
mitigation algorithms intended to support redundant servers and
diverse network paths, it should be operated only in conjunction with
a source of external synchronization, such as a reliable radio clock
or telephone modem. In this case it operates as a primary (stratum
A SNTP server can operate with any unicast, manycast, or broadcast
address or any combination of these addresses. A unicast or manycast
server receives a request (NTP mode 3), modifies certain fields in
the NTP header, and sends a reply (NTP mode 4), possibly using the
same message buffer as the request. A manycast server listens on the
designated broadcast address, but uses its own unicast IP address in
the source address field of the reply. Other than the selection of
address in the reply, the operations of manycast and unicast servers
are identical. Broadcast messages are normally sent at intervals
from 64 s to 1024 s, depending on the expected frequency tolerance of
the client clocks and the required accuracy.
Unicast and manycast servers copy the VN and Poll fields of the
request intact to the reply and set the Stratum field to 1.
Note that SNTP servers normally operate as primary (stratum 1)
servers. Although operating at higher strata (up to 15) while
synchronizing to an external source such as a GPS receiver is not
forbidden, this is strongly discouraged.
If the Mode field of the request is 3 (client), the reply is set to 4
(server). If this field is set to 1 (symmetric active), the reply is
set to 2 (symmetric passive). This allows clients configured in
either client (NTP mode 3) or symmetric active (NTP mode 1) to
interoperate successfully, even if configured in possibly suboptimal
ways. For any other value in the Mode field, the request is
discarded. In broadcast (unsolicited) mode, the VN field is set to
4, the Mode field is set to 5 (broadcast), and the Poll field set to
the nearest integer base-2 logarithm of the poll interval.
Note that it is highly desirable that a broadcast server also
supports unicast clients. This is so a potential broadcast client
can calculate the propagation delay using a client/server exchange
prior to switching to broadcast client (listen-only) mode. By
design, a manycast server is also a unicast server. There does
not seem to be a great advantage for a server to operate as both
broadcast and manycast at the same time, although the protocol
specification does not forbid it.
A broadcast or manycast server does not send packets if not
synchronized to a correctly operating reference source. It may or
may not respond to a client request if it is not synchronized, but
the preferred option is to respond because this allows reachability
to be determined regardless of synchronization state. If the server
has never synchronized to a reference source, the LI field is set to
3 (unsynchronized). Once synchronized to a reference source, the LI
field is set to one of the other three values and remains at the last
value set even if the reference source becomes unreachable or turns
If the server is synchronized to a reference source, the Stratum
field is set to 1, and the Reference Identifier field is set to the
ASCII source identifier shown in Figure 2. If the server is not
synchronized, the Stratum field is set to zero, and the Reference
Identifier field is set to an ASCII error identifier described below.
The Precision field is set to reflect the maximum reading error of
the system clock. For all practical cases it is computed as the
negative base-2 logarithm 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 0 for a primary server.
The timestamp fields in the server message are set as follows. If
the server is unsynchronized or first coming up, all timestamp fields
are set to zero, with one exception. If the message is a reply to a
previously received client request, the Transmit Timestamp field of
the request is copied unchanged to the Originate Timestamp field of
the reply. It is important that this field be copied intact, as an
NTP or SNTP client uses it to avoid bogus messages.
If the server is synchronized, the Reference Timestamp is set to the
time the last update was received from the reference source. The
Originate Timestamp field is set as in the unsynchronized case above.
The Transmit Timestamp field is set to the time of day when the
message is sent. In broadcast messages the Receive Timestamp field
is set to zero and copied from the Transmit Timestamp field in other
messages. The following table summarizes these actions.
Field Name Unicast/Manycast Broadcast
LI ignore as needed as needed
VN 1-4 copied from 4
Mode 3 4 5
Stratum ignore 1 1
Poll ignore copied from log2 poll
Precision ignore -log2 server -log2 server
Root Delay ignore 0 0
Root Dispersion ignore 0 0
Reference Identifier ignore source ident source ident
Reference Timestamp ignore time of last time of last
source update source update
Originate Timestamp ignore copied from 0
Receive Timestamp ignore time of day 0
Transmit Timestamp (see text) time of day time of day
Authenticator optional optional optional
There is some latitude on the part of most clients to forgive invalid
timestamps, such as might occur when the server is first coming up or
during periods when the reference source is inoperative. The most
important indicator of an unhealthy server is the Stratum field, in
which a value of 0 indicates an unsynchronized condition. When this
value is displayed, clients should discard the server message,
regardless of the contents of other fields.
7. Configuration and Management
Initial setup for SNTP servers and clients can be done using a web
client, if available, or a serial port, if not. Some folks hoped
that in-service management of NTP and SNTPv4 servers and clients
could be performed using SNMP and a suitable MIB to be published, and
this has happened in some commercial SNTP servers. But, the means
that have been used in the last two decades and probably will be used
in the next is the NTP control and monitoring protocol defined in RFC
1305. Ordinarily, SNTP servers and clients are expected to operate
with little or no site-specific configuration, other than specifying
the client IP address, subnet mask, and gateway.
Unicast clients must be provided with one or more designated server
names or IP addresses. If more than one server is provided, one can
be used for active operation and one of the others for backup should
the active one fail or show an error condition. It is not normally
useful to use more than one server at a time, as with millions of
SNTP-enabled devices expected in the near future, such use would
represent unnecessary drain on network and server resources.
Broadcast servers and manycast clients must be provided with the TTL
and local broadcast or multicast group address. Unicast and manycast
servers and broadcast clients may be configured with a list of
address-mask pairs for access control, so that only those clients or
servers known to be trusted will be accepted. Multicast servers and
clients must implement the IGMP protocol and be provided with the
local broadcast or multicast group address as well. The
configuration data for cryptographic authentication is beyond the
scope of this memo.
There are several scenarios that provide automatic server discovery
and selection for SNTP clients with no pre-specified server
configuration. For instance, a role server with CNAME such as
pool.ntp.org returns a randomized list of volunteer secondary server
addresses, and the client can select one or more as candidates. For
an IP subnet or LAN segment including an NTP or SNTP server, SNTP
clients can be configured as broadcast clients. The same approach
can be used with multicast servers and clients. In both cases,
provision of an access control list is a good way to ensure that only
trusted sources can be used to set the system clock.
In another scenario suitable for an extended network with significant
network propagation delays, clients can be configured for manycast
addresses, both upon initial startup and after some period when the
currently selected unicast source has not been heard. Following the
defined protocol, the client binds to the server from which the first
reply is received and continues operation in unicast mode.
8. The Kiss-o'-Death Packet
In the rambunctious Internet of today, it is imperative that some
means be available to tell a client to stop making requests and to go
somewhere else. A recent experience involved a large number of
home/office routers all configured to use a particular university
time server. Under some error conditions, a substantial fraction of
these routers would send packets at intervals of one second. The
resulting traffic spike was dramatic, and extreme measures were
required to diagnose the problem and to bring it under control. The
conclusion is that clients must respect the means available to
targeted servers to stop them from sending packets.
According to the NTP specification RFC 1305, if the Stratum field in
the NTP header is 1, indicating a primary server, the Reference
Identifier field contains an ASCII string identifying the particular
reference clock type. However, in RFC 1305 nothing is said about the
Reference Identifier field if the Stratum field is 0, which is called
out as "unspecified". However, if the Stratum field is 0, the
Reference Identifier field can be used to convey messages useful for
status reporting and access control. In NTPv4 and SNTPv4, packets of
this kind are called Kiss-o'-Death (KoD) packets, and the ASCII
messages they convey are called kiss codes. The KoD packets got
their name because an early use was to tell clients to stop sending
packets that violate server access controls.
In general, an SNTP client should stop sending to a particular server
if that server returns a reply with a Stratum field of 0, regardless
of kiss code, and an alternate server is available. If no alternate
server is available, the client should retransmit using an
exponential-backoff algorithm described in the next section.
The kiss codes can provide useful information for an intelligent
client. These codes are encoded in four-character ASCII strings left
justified and zero filled. The strings are designed for character
displays and log files. Usually, only a few of these codes can occur
with SNTP clients, including DENY, RSTR, and RATE. Others can occur
more rarely, including INIT and STEP, when the server is in some
special temporary condition. Figure 3 shows a list of the kiss codes
currently defined. These are for informational purposes only; the
list might be modified or extended in the future.
ACST The association belongs to a anycast server
AUTH Server authentication failed
AUTO Autokey sequence failed
BCST The association belongs to a broadcast server
CRYP Cryptographic authentication or identification failed
DENY Access denied by remote server
DROP Lost peer in symmetric mode
RSTR Access denied due to local policy
INIT The association has not yet synchronized for the first
MCST The association belongs to a manycast server
NKEY No key found. Either the key was never installed or
is not trusted
RATE Rate exceeded. The server has temporarily denied access
because the client exceeded the rate threshold
RMOT Somebody is tinkering with the association from a remote
host running ntpdc. Not to worry unless some rascal has
stolen your keys
STEP A step change in system time has occurred, but the
association has not yet resynchronized
Figure 3. Kiss Codes9. On Being a Good Network Citizen
SNTP and its big brother NTP have been in explosive growth over the
last few years, mirroring the growth of the Internet. Just about
every Internet appliance has some kind of NTP support, including
Windows XP, Cisco routers, embedded controllers, and software systems
of all kinds. This is the first edition of the SNTP RFC where it has
become necessary to lay down rules of engagement in the form of
design criteria for SNTP client implementations. This is necessary
to educate software developers regarding the proper use of Internet
time server resources as the Internet expands and demands on time
servers increase, and to prevent the recurrence of the sort of
problem mentioned above.
10. Best Practices
NTP and SNTP clients can consume considerable network and server
resources if they are not good network citizens. There are now
consumer Internet commodity devices numbering in the millions that
are potential customers of public and private NTP and SNTP servers.
Recent experience strongly suggests that device designers pay
particular attention to minimizing resource impacts, especially if
large numbers of these devices are deployed. The most important
design consideration is the interval between client requests, called
the poll interval. It is extremely important that the design use the
maximum poll interval consistent with acceptable accuracy.
1. A client MUST NOT under any conditions use a poll interval less
than 15 seconds.
2. A client SHOULD increase the poll interval using exponential
backoff as performance permits and especially if the server does
not respond within a reasonable time.
3. A client SHOULD use local servers whenever available to avoid
unnecessary traffic on backbone networks.
4. A client MUST allow the operator to configure the primary and/or
alternate server names or addresses in addition to or in place of
a firmware default IP address.
5. If a firmware default server IP address is provided, it MUST be a
server operated by the manufacturer or seller of the device or
another server, but only with the operator's permission.
6. A client SHOULD use the Domain Name System (DNS) to resolve the
server IP addresses, so the operator can do effective load
balancing among a server clique and change IP address binding to
7. A client SHOULD re-resolve the server IP address at periodic
intervals, but not at intervals less than the time-to-live field
in the DNS response.
8. A client SHOULD support the NTP access-refusal mechanism so that
a server kiss-o'-death reply in response to a client request
causes the client to cease sending requests to that server and to
switch to an alternate, if available.
The following algorithm can be used as a pattern for specific
implementations. It uses the following variables:
Timer: This is a counter that decrements at a fixed rate. When it
reaches zero, a packet is sent, and the timer is initialized with the
timeout for the next packet.
Maximum timeout: This is the maximum timeout determined from the
given oscillator frequency tolerance and the required accuracy.
Server Name: This is the DNS name of the server. There may be more
than one of them, to be selected by some algorithm not considered
Server IP Address: This is the IPv4, IPv6, or OSI address of the
If the firmware or documentation includes specific server names, the
names should be those the manufacturer or seller operates as a
customer convenience or those for which specific permission has been
obtained from the operator. A DNS request for a generic server name,
such as ntp.mytimeserver.com, should result in a random selection of
server IP addresses available for that purpose. Each time a DNS
request is received, a new randomized list is returned. The client
ordinarily uses the first address on the list.
When candidate SNTP or NTP servers are selected, it is imperative
to respect the server operator's conditions of access. Lists of
public servers and their conditions of access are available at
www.ntp.org. A semi-automatic server discovery scheme using DNS
is described at that site. Some ISPs operate public servers,
although finding them via their help desks can be difficult.
A well-behaved client operates as follows (note that steps 2-4
constitute a synchronization loop):
1. Consider the specified frequency tolerance of the system clock
oscillator. Define the required accuracy of the system clock,
then calculate the maximum timeout. For instance, if the
frequency tolerance is 200 parts per million (PPM) and the
required accuracy is one minute, the maximum timeout is about 3.5
days. Use the longest maximum timeout possible given the system
constraints to minimize time server aggregate load, but never
make it less than 15 minutes.
2. When the client is first coming up or after reset, randomize the
timeout from one to five minutes. This is to minimize shock when
3000 PCs are rebooted at the same time power is restored after a
blackout. Assume at this time that the IP address is unknown and
that the system clock is unsynchronized. Otherwise, use the
timeout value as calculated in previous loop steps. Note that it
may be necessary to refrain from implementing the aforementioned
random delay for some classes of International Computer Security
Association (ICSA) certification.
3. When the timer reaches zero, if the IP address is not known, send
a DNS query packet; otherwise, send an NTP request packet to that
address. If no reply packet has been heard since the last
timeout, double the timeout, but do not make it greater than the
maximum timeout. If primary and secondary time servers have been
configured, alternate queries between the primary and secondary
servers when no successful response has been received.
4. If a DNS reply packet is received, save the IP address and
continue at step 2. If a KoD packet is received, remove that
time server from the list, activate the secondary time server,
and continue at step 2. If a received packet fails the sanity
checks, drop that packet and also continue at step 2. If a valid
NTP packet is received, update the system clock, set the timeout
to the maximum, and continue at step 2.
11. Security Considerations
Without cryptographic authentication, SNTPv4 service is vulnerable to
disruption by misbehaving or hostile SNTP or NTP broadcast servers
elsewhere in the Internet. It is strongly recommended that access
controls and/or cryptographic authentication means be provided for
additional security. This document includes protocol provisions for
adding such security mechanisms, but it does not define the
mechanisms themselves. A separate document [MIL03] in preparation
will define a cryptographic security mechanism for SNTP.
Jeff Learman was helpful in developing the OSI model for this
protocol. Ajit Thyagarajan provided valuable suggestions and
14. Informative References
[BRA97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[COL94] Colella, R., Callon, R., Gardner, E., and Y. Rekhter,
"Guidelines for OSI NSAP Allocation in the Internet", RFC
1629, May 1994.
[DEE89] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[DEE98] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[DOB91] Shue, C., Haggerty, W., and K. Dobbins, "OSI connectionless
transport services on top of UDP: Version 1", RFC 1240, June
[FUR94] Furniss, P., "Octet Sequences for Upper-Layer OSI to Support
Basic Communications Applications", RFC 1698, October 1994.
[ISO86] International Standards 8602 - Information Processing
Systems - OSI: Connectionless Transport Protocol
Specification. International Standards Organization,
[MIL92] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation and Analysis", RFC 1305, March 1992.
[MIL03] Mills, D., "The Autokey Security Architecture, Protocol and
stime/stime.pdf, August 2003.
[PAR93] Partridge, C., Mendez, T., and W. Milliken, "Host Anycasting
Service", RFC 1546, November 1993.
[POS80] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
[POS83] Postel, J. and K. Harrenstien, "Time Protocol", STD 26, RFC
868, May 1983.
[SRI99] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC 2663,
[SRI01] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
David L. Mills
Electrical and Computer Engineering Department
University of Delaware
Newark, DE 19716
Phone: (302) 831-8247
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