rfc1305.txt

RFC 的详细文档！

All mathematical operations expressed or implied herein are in two's-
complement, fixed-point arithmetic. Data are specified as integer or
fixed-point quantities, with bits numbered in big-endian fashion from
zero starting at the left, or high-order, position. Since various
implementations may scale externally derived quantities for internal
use, neither the precision nor decimal-point placement for fixed-point
quantities is specified. Unless specified otherwise, all quantities are
unsigned and may occupy the full field width with an implied zero
preceding bit zero. Hardware and software packages designed to work with
signed quantities will thus yield surprising results when the most
significant (sign) bit is set. It is suggested that externally derived,
unsigned fixed-point quantities such as timestamps be shifted right one
bit for internal use, since the precision represented by the full field
width is seldom justified.

Since NTP timestamps are cherished data and, in fact, represent the main
product of the protocol, a special timestamp format has been
established. NTP timestamps are represented as a 64-bit unsigned fixed-
point number, in seconds relative to 0h on 1 January 1900. The integer
part is in the first 32 bits and the fraction part in the last 32 bits.
This format allows convenient multiple-precision arithmetic and
conversion to Time Protocol representation (seconds), but does
complicate the conversion to ICMP Timestamp message representation
(milliseconds). The precision of this representation is about 200
picoseconds, which should be adequate for even the most exotic
requirements.

Timestamps are determined by copying the current value of the local
clock to a timestamp when some significant event, such as the arrival of
a message, occurs. In order to maintain the highest accuracy, it is
important that this be done as close to the hardware or software driver
associated with the event as possible. In particular, departure
timestamps should be redetermined for each link-level retransmission. In
some cases a particular timestamp may not be available, such as when the
host is rebooted or the protocol first starts up. In these cases the 64-
bit field is set to zero, indicating the value is invalid or undefined.

Note that since some time in 1968 the most significant bit (bit 0 of the
integer part) has been set and that the 64-bit field will overflow some
time in 2036. Should NTP be in use in 2036, some external means will be
necessary to qualify time relative to 1900 and time relative to 2036
(and other multiples of 136 years). Timestamped data requiring such
qualification will be so precious that appropriate means should be
readily available. There will exist an 200-picosecond interval,
henceforth ignored, every 136 years when the 64-bit field will be zero
and thus considered invalid.

State Variables and Parameters

Following is a summary of the various state variables and parameters
used by the protocol. They are separated into classes of system
variables, which relate to the operating system environment and local-
clock mechanism; peer variables, which represent the state of the
protocol machine specific to each peer; packet variables, which
represent the contents of the NTP message; and parameters, which
represent fixed configuration constants for all implementations of the
current version. For each class the description of the variable is
followed by its name and the procedure or value which controls it. Note
that variables are in lower case, while parameters are in upper case.
Additional details on formats and use are presented in later sections
and Appendices.

Common Variables

The following variables are common to two or more of the system, peer
and packet classes. Additional variables are specific to the optional
authentication mechanism as described in Appendix C. When necessary to
distinguish between common variables of the same name, the variable
identifier will be used.

Peer Address (peer.peeraddr, pkt.peeraddr), Peer Port (peer.peerport,
pkt.peerport): These are the 32-bit Internet address and 16-bit port
number of the peer.

Host Address (peer.hostaddr, pkt.hostaddr), Host Port (peer.hostport,
pkt.hostport): These are the 32-bit Internet address and 16-bit port
number of the host. They are included among the state variables to
support multi-homing.

Leap Indicator (sys.leap, peer.leap, pkt.leap): This is a two-bit code
warning of an impending leap second to be inserted in the NTP timescale.
The bits are set before 23:59 on the day of insertion and reset after
00:00 on the following day. This causes the number of seconds (rollover
interval) in the day of insertion to be increased or decreased by one.
In the case of primary servers the bits are set by operator
intervention, while in the case of secondary servers the bits are set by
the protocol. The two bits, bit 0 and bit 1, respectively, are coded as
follows:
@Z_TBL_BEG = COLUMNS(2), DIMENSION(IN), COLWIDTHS(E1,E8), WIDTH(5.0000),
ABOVE(.0830), BELOW(.0830), HGUTTER(.0560), KEEP(OFF), ALIGN(CT)

@Z_TBL_BODY = TABLE TEXT, TABLE TEXT

00, no warning

01, last minute has 61 seconds

10, last minute has 59 seconds

11, alarm condition (clock not synchronized)

@Z_TBL_END =

In all except the alarm condition (112), NTP itself does nothing with
these bits, except pass them on to the time-conversion routines that are
not part of NTP. The alarm condition occurs when, for whatever reason,
the local clock is not synchronized, such as when first coming up or
after an extended period when no primary reference source is available.

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

@Z_TBL_BEG = COLUMNS(2), DIMENSION(IN), COLWIDTHS(E1,E8), WIDTH(5.0000),
ABOVE(.0830), BELOW(.0830), HGUTTER(.0560), KEEP(OFF), ALIGN(CT)

@Z_TBL_BODY = TABLE TEXT, TABLE TEXT

0, unspecified

1, symmetric active

2, symmetric passive

3, client

4, server

5, broadcast

6, reserved for NTP control messages

7, reserved for private use

@Z_TBL_END =

Stratum (sys.stratum, peer.stratum, pkt.stratum): This is an integer
indicating the stratum of the local clock, with values defined as
follows:

@Z_TBL_BEG = COLUMNS(2), DIMENSION(IN), COLWIDTHS(E1,E8), WIDTH(5.0000),
ABOVE(.0830), BELOW(.0830), HGUTTER(.0560), KEEP(OFF), ALIGN(CT)

@Z_TBL_BODY = TABLE TEXT, TABLE TEXT

0, unspecified

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

2-255, secondary reference (via NTP)

@Z_TBL_END =

For comparison purposes a value of zero is considered greater than any
other value. Note that the maximum value of the integer encoded as a
packet variable is limited by the parameter NTP.MAXSTRATUM.

Poll Interval (sys.poll, peer.hostpoll, peer.peerpoll, pkt.poll): This
is a signed integer indicating the minimum interval between transmitted
messages, in seconds as a power of two. For instance, a value of six
indicates a minimum interval of 64 seconds.

Precision (sys.precision, peer.precision, pkt.precision): This is a
signed integer indicating the precision of the various clocks, in
seconds to the nearest power of two. The value must be rounded to the
next larger power of two; for instance, a 50-Hz (20 ms) or 60-Hz (16.67
ms) power-frequency clock would be assigned the value -5 (31.25 ms),
while a 1000-Hz (1 ms) crystal-controlled clock would be assigned the
value -9 (1.95 ms).

Root Delay (sys.rootdelay, peer.rootdelay, pkt.rootdelay): This is a
signed fixed-point number indicating the total roundtrip delay to the
primary reference source at the root of the synchronization subnet, in
seconds. Note that this variable can take on both positive and negative
values, depending on clock precision and skew.

Root Dispersion (sys.rootdispersion, peer.rootdispersion,
pkt.rootdispersion): This is a signed fixed-point number indicating the
maximum error relative to the primary reference source at the root of
the synchronization subnet, in seconds. Only positive values greater
than zero are possible.

Reference Clock Identifier (sys.refid, peer.refid, pkt.refid): This is a
32-bit code identifying the particular reference clock. In the case of
stratum 0 (unspecified) or stratum 1 (primary reference source), this is
a four-octet, left-justified, zero-padded ASCII string, for example (see
Appendix A for comprehensive list):

@Z_TBL_BEG = COLUMNS(3), DIMENSION(IN), COLWIDTHS(E2,E2,E5),
WIDTH(4.1700), ABOVE(.1670), BELOW(.0830), HGUTTER(.3330),
BOX(Z_SINGLE), KEEP(ON), ALIGN(CT), L1(R1C0..R1C3)

@Z_TBL_BODY = TABLE CENTER, TABLE HEADER, TABLE HEADER

Stratum, Code, Meaning

@Z_TBL_BODY = TABLE CENTER, TABLE TEXT, TABLE TEXT

0, DCN, DCN routing protocol

0, TSP, TSP time protocol

1, ATOM, Atomic clock (calibrated)

1, WWVB, WWVB LF (band 5) radio

1, GOES, GOES UHF (band 9) satellite

@Z_TBL_BODY = TABLE CENTER, TABLE HEADER, TABLE HEADER

1, WWV, WWV HF (band 7) radio

@Z_TBL_END =

In the case of stratum 2 and greater (secondary reference) this is the
four-octet Internet address of the peer selected for synchronization.

Reference Timestamp (sys.reftime, peer.reftime, pkt.reftime): This is
the local time, in timestamp format, when the local clock was last
updated. If the local clock has never been synchronized, the value is
zero.

Originate Timestamp (peer.org, pkt.org): This is the local time, in
timestamp format, at the peer when its latest NTP message was sent. If
the peer becomes unreachable the value is set to zero.

Receive Timestamp (peer.rec, pkt.rec): This is the local time, in
timestamp format, when the latest NTP message from the peer arrived. If
the peer becomes unreachable the value is set to zero.

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

System Variables

Table 1<$&tab1> shows the complete set of system variables. In addition
to the common variables described previously, the following variables
are used by the operating system in order to synchronize the local
clock.

Local Clock (sys.clock): This is the current local time, in timestamp
format. Local time is derived from the hardware clock of the particular
machine and increments at intervals depending on the design used. An
appropriate design, including slewing and skew-Compensation mechanisms,
is described in Section 5.

Clock Source (sys.peer): This is a selector identifying the current
synchronization source. Usually this will be a pointer to a structure
containing the peer variables. The special value NULL indicates there is
no currently valid synchronization source.

Peer Variables

Table 2 shows the complete set of peer variables. In addition to the
common variables described previously, the following variables are used
by the peer management and measurement functions.

Configured Bit (peer.config): This is a bit indicating that the
association was created from configuration information and should not be
demobilized if the peer becomes unreachable.

Update Timestamp (peer.update): This is the local time, in timestamp
format, when the most recent NTP message was received. It is used in
calculating the skew dispersion.

Reachability Register (peer.reach): This is a shift register of
NTP.WINDOW bits used to determine the reachability status of the peer,
with bits entering from the least significant (rightmost) end. A peer is
considered reachable if at least one bit in this register is set to one.

Peer Timer (peer.timer): This is an integer counter used to control the
interval between transmitted NTP messages. Once set to a nonzero value,
the counter decrements at one-second intervals until reaching zero, at
which time the transmit procedure is called. Note that the operation of
this timer is independent of local-clock updates, which implies that the
timekeeping system and interval-timer system architecture must be
independent of each other.<$&tab2>