Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI
draft-mills-sntp-v4-00
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 4330.
|
|
|---|---|---|---|
| Author | Professor David L. Mills | ||
| Last updated | 2020-01-21 (Latest revision 2003-09-17) | ||
| RFC stream | Independent Submission | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Stream | ISE state | (None) | |
| Consensus boilerplate | Unknown | ||
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 4330 (Informational) | |
| Action Holders |
(None)
|
||
| Telechat date | (None) | ||
| Responsible AD | Dr. Thomas Narten | ||
| Send notices to | karen.odonoghue@navy.mil |
draft-mills-sntp-v4-00
INTERNET DRAFT D. Mills
Network Working Group University of Delaware
Obsoletes: 2030, 1769 D. Plonka
Category: Informational University of Wisconsin
J. Montgomery
Netgear
September 2003
Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI
<draft-mills-sntp-v4-00.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsolete by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This memorandum describes the Simple Network Time Protocol (SNTP)
Version 4, which is a subset 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. SNTP Version 4 clarifies
certain design features of NTP which 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.
The only significant protocol change in SNTP Version 4 is a modified
header interpretation to accommodate Internet Protocol Version 6
(IPv6) (RFC-1883) and OSI (RFC-1629) addressing. However, SNTP
Version 4 includes an anycast mode and a public-key based
authentication scheme designed specifically for broadcast and anycast
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applications. The authentication scheme extension is described in
another RFC. Until a definitive specification is published, these
extensions 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.
This memorandum obsoletes RFC-1769, which describes SNTP Version 3,
and RFC-2030, which describes SNTP Version 4.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Operating Modes and Addressing . . . . . . . . . . . . . . . . 5
3. NTP Timestamp Format . . . . . . . . . . . . . . . . . . . . . 6
4. Message Format . . . . . . . . . . . . . . . . . . . . . . . . 8
5. SNTP Client Operations . . . . . . . . . . . . . . . . . . . . 12
6. SNTP Server Operations . . . . . . . . . . . . . . . . . . . . 16
7. Configuration and Management . . . . . . . . . . . . . . . . . 19
8. The Kiss-o'-Death Packet . . . . . . . . . . . . . . . . . . . 20
9. On Being a Good Network Citizen. . . . . . . . . . . . . . . . 21
10. Best Practices . . . . . . . . . . . . . . . . . . . . . . . . 22
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
12. Informative References . . . . . . . . . . . . . . . . . . . . 24
13. Security Considerations. . . . . . . . . . . . . . . . . . . . 26
14. Author's Address . . . . . . . . . . . . . . . . . . . . . . . 26
15. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 27
1. 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, in addition, engineered
algorithms to improve the timekeeping quality and mitigate among
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 in 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
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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. While 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
accuracy expectations.
This memo describes the Simple Network Time Protocol Version 4
(SNTPv4), which is a simplified access paradigm for servers and
clients using NTP and SNTP, current and previous versions. 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.
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 a NTP or SNTP server, NTP and SNTP
clients are indistinguishable; to a 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
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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 which provide for IPv6 and OSI 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, while
secondary servers use the 32-bit IPv4 address of the synchronization
source. In SNTP version 4 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
as SNTPv4.
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 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 inadvisable.
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
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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.
2. Operating Modes and Addressing
Unless excepted in context, reference to broadcast address means IPv4
broadcast address, IPv4 multicast group address or IPv6 site-local
scope address. 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
anycast (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.
Anycast is designed for use with a set of cooperating servers whose
addresses are not known beforehand. The anycast client sends an
ordinary NTP client request to a designated broadcast address. One
or more anycast servers listen on that address. Upon receiving a
request, an anycast server sends an ordinary NTP server reply to the
client. The client then binds to the server from which the first
such message was received and continues operation with that unicast
addresses. Subsequent replies from other anycast servers are
ignored.
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 224.0.1.1 and the IPv6 group address
ending :101, with prefix determined by scoping rules. The NTP
broadcast address for OSI has yet to be determined. Notwithstanding
the IANA reserved addresses, other multicast addresses can be used
which do not conflict with others assigned in scope. In the case of
IPv4 multicast or IPv6 broadcast addresses, the client must implement
the Internet Group Management Protocol (IGMP) as described in RFC-
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3376 [CAIN02], in order that the local router joins the multicast
group and relays messages to the IPv4 or IPv6 multicast group. The
scoping, routing and group membership procedures are determined by
considerations beyond the scope of this memo.
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 anycast servers in scope will
reply to a client request. The engineering principles which
determine the proper values to be used are beyond the scope of
this memo.
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.
While not integral to the SNTP specification, 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, while IP
multicast group addresses will be used only in cases where the TTL
is engineered specifically for each service domain.
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.
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. In the fraction part, the non-significant low order
bits are not specified and ordinarily set to 0.
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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 as a means of loop detection and
replay detection (see below). It is important that the bitstring
be unpredictable by a 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
remains a possibility that it will be in use 33 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.
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4. Message Format
Both NTP and SNTP are clients of the User Datagram Protocol (UDP)
specified in RFC-768 [POS80], which itself is a client of the
Internet Protocol (IP) specified in RFC-791) [DAR81]. The structure
of the IP and UDP headers is 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 also for
compatibility with Network Address Port Translation (NAPT)
described in RFC-2663 [SRI99] and RFC-3022 [SRI02].
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.
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. This field is significant only in server messages, where the
values are defined as follows:
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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) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Identifier (optional) (32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| Message Digest (optional) (128) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. NTP Packet Header
LI Meaning
---------------------------------------------
0 no warning
1 last minute has 61 seconds
2 last minute has 59 seconds)
3 alarm condition (clock not synchronized)
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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 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
inspected.
Mode: This is a three-bit number indicating the protocol mode. The
values are 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
In unicast and anycast 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
clients.
Stratum: This is a eight-bit unsigned integer indicating the stratum.
This field is significant only in SNTP server messages, where the
values are defined as follows:
Stratum Meaning
----------------------------------------------
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)
16-255 reserved
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).
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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 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 milliseconds.
Root Dispersion: This is a 32-bit unsigned fixed-point number
indicating the nominal error relative to the primary reference
source, in seconds with 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.
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
source
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 Kaui 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
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
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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
source.
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, since 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 significant only in server
messages, where the value is the time at which 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
format.
Transmit Timestamp: This is the time at which the request departed
the client or the reply departed the server, in 64-bit timestamp
format.
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
A SNTP client can operate in unicast, broadcast or anycast 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 anycast 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 anycast
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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
anycast and unicast clients are identical.
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
Section 9.
A unicast or anycast 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 of 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
anycast requests. They set the VN field to any version number
supported by the server selected by configuration or discovery and
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 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.
While not necessary in a conforming client implementation, in unicast
and anycast modes it highly recommended that the Transmit Timestamp
field in the request is set to the time of day according to the
client clock in NTP timestamp format. 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 to verify that the server reply is in fact a legitimate
response to the specific client request and avoid replays. In
broadcast mode, the client has no information to calculate the
propagation delay or 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
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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 local 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 in general 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, anycast 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.
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Field Name Unicast/Anycast Broadcast
Request Reply
---------------------------------------------------------------
LI 0 0-3 0-3
VN 1-4 copied from 1-4
request
Mode 1 or 3 2 or 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
While 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
suggested checks.
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
server reply.
3. The Originate Timestamp in the server reply should match the
Transmit Timestamp used in the client request.
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4. The server reply should be discarded if any of the LI, Stratum, or
Transmit Timestamp fields are 0 or the Mode field is not 4
(unicast) or 5 (broadcast).
5. A truly paranoid client can check 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 16
seconds. 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 a NTP or SNTP client of the same
or previous versions retains no persistent state. Since a SNTP
server ordinarily does not implement the full suite of grooming and
mitigation algorithms intended to support redundant servers and
diverse network paths, a SNTP server 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 1) server.
A SNTP server can operate with any unicast, anycast or broadcast
address or any combination of these addresses. A unicast or anycast
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 anycast 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 anycast and unicast servers
are identical. Broadcast messages are normally sent at poll
intervals from 64 s to 1024 s, depending on the expected frequency
tolerance of the client clocks and the required accuracy.
Unicast and anycast 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. While operating at higher strata (up to 15) and at the
same time 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
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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. A
anycast server by design also is a unicast server. There does not
seem to be a great advantage for a server to operate as both
broadcast and anycast at the same time, although the protocol
specification does not forbid it.
A broadcast or anycast server may or may not respond if not
synchronized to a correctly operating reference source, but the
preferred option is to respond, since 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
faulty.
If 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 not synchronized, the Stratum field
is set to zero and the Reference Identifier field set to an ASCII
error identifier described below. In broadcast mode, the server
sends broadcasts only if synchronized to a correctly operating
reference source.
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;
optionally, the Root Dispersion field can be set to a value
corresponding to the maximum expected error of the radio clock
itself.
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.
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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 are 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/Anycast Broadcast
Request Reply
----------------------------------------------------------------
LI ignore as needed as needed
VN 1-4 copied from 4
request
Mode 1 or 3 2 or 4 5
Stratum ignore 1 1
Poll ignore copied from log2 poll
request interval
Precision ignore -log2 server -log2 server
significant significant
bits bits
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
transmit
timestamp
Receive Timestamp ignore time of day 0
Transmit Timestamp (see text) time of day time of day
Authenticator optional optional optional
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There is some latitude on the part of most clients to forgive invalid
timestamps, such as might occur when 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 be
performed using SNMP and a suitable MIB to be published, and this has
happened in some commercial SNTP servers. But, the means used in the
last decade and probably 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, 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 anycast clients must be provided with the TTL
and local broadcast or multicast group address. Unicast and anycast
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 which 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 a NTP or SNTP server, SNTP
clients can be configured as broadcast clients. The same approach
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can be used with multicast servers and clients. In both cases,
provision of an access control list is a good way to insure 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 anycast
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 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 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.
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Code Meaning
--------------------------------------------------------------
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
time
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 Codes
In general, a 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.
9. 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
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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 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 one minute.
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
canonical names.
7. A client SHOULD re-resolve the server IP address on a periodic
intervals, but not less than the time-to-live field in the DNS
response.
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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 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
here.
Server IP Address: This is the IPv4, IPv6 or OSI address of the
server.
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 results 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 selecting candidate SNTP or NTP servers, 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 helpdesks can be difficult.
A well behaved client operates as follows (note that steps 2 - 4
comprise 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
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days. Use the longest maximum timeout possible given the system
constraints to minimize time server aggregate load, but never less
than 15 minutes.
2. When 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 the IP address is unknown and 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 ICSA certification.
3. When the timer reaches zero, if the IP address is not known, send
a DNS query packet; otherwise send a NTP request packet to that
address. If no reply packet has been heard since the last
timeout, double the timeout, but not 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 in step 2. If a KoD packet is received remove that time
server from the list, activate the secondary time server and
continue in step 2. If a received packet fails the sanity checks,
drop that packet and also continue in step 2. If a valid NTP
packet is received, update the system clock, set the timeout to
the maximum, and continue to step 2.
11. Acknowledgements
Jeff Learman was helpful in developing the OSI model for this
protocol. Ajit Thyagarajan provided valuable suggestions and
corrections.
12. Informative References
[CAIN02] Cain, B., Deering, S., Kouvalas, I., Fenner, B. and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, Cereva Networks, October 2002.
[COL94] Colella, R., Callon, R., Gardner, E. and Y. Rekhter,
"Guidelines for OSI NSAP Allocation in the Internet", RFC
1629, May 1994.
[DAR81] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
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Internet-Draft SNTPv4 for IPv4, IPv6 and OSI September 2003
[DEE89] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[DEE96] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 1883, January 1996.
[DOB91] Dobbins, K, Haggerty, W. and C. Shue, "OSI connectionless
transport services on top of UDP - Version: 1", RFC 1240,
June 1991.
[EAS95] Eastlake, D., 3rd., and C. Kaufman, "Domain Name System
Security Extensions", Work in Progress.
[FUR94] Furniss, P., "Octet Sequences for Upper-Layer OSI to Support
Basic Communications Applications", RFC 1698, October 1994.
[HIN96] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 1884, January 1996.
[HIN03] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[ISO86] International Standards 8602 - Information Processing
Systems - OSI: Connectionless Transport Protocol
Specification. International Standards Organization,
December 1986.
[MIL92] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation", RFC 1305, March 1992.
[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
1980.
[POS83] Postel, J., "Time Protocol", STD 26, RFC 868, May 1983.
[SRI99] Srisuresh, P. and M. Holdrege. "IP Network Address
Translator (NAT) Terminology and Considerations", RFC 2663,
August 1999.
[SRI01] Srisuresh, P. and K. Egevang. "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
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13. Security Considerations
Security issues are not discussed in this memo.
14. Author's Address
David L. Mills
Electrical and Computer Engineering Department
University of Delaware
Newark, DE 19716
Phone: (302) 831-8247
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15. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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