util.io
Class NtpMessage

java.lang.Object
  util.io.NtpMessage

public class NtpMessage
extends java.lang.Object

This class represents a NTP message, as specified in RFC 2030. The message format is compatible with all versions of NTP and SNTP. This class does not support the optional authentication protocol, and ignores the key ID and message digest fields. For convenience, this class exposes message values as native Java types, not the NTP-specified data formats. For example, timestamps are stored as doubles (as opposed to the NTP unsigned 64-bit fixed point format). However, the contructor NtpMessage(byte[]) and the method toByteArray() allow the import and export of the raw NTP message format. Usage example // Send message DatagramSocket socket = new DatagramSocket(); InetAddress address = InetAddress.getByName("ntp.cais.rnp.br"); byte[] buf = new NtpMessage().toByteArray(); DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 123); socket.send(packet); // Get response socket.receive(packet); System.out.println(msg.toString()); This code is copyright (c) Adam Buckley 2004 This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. A HTML version of the GNU General Public License can be seen at http://www.gnu.org/licenses/gpl.html This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. Comments for member variables are taken from RFC2030 by David Mills, University of Delaware. Number format conversion code in NtpMessage(byte[] array) and toByteArray() inspired by http://www.pps.jussieu.fr/~jch/enseignement/reseaux/ NTPMessage.java which is copyright (c) 2003 by Juliusz Chroboczek

Author:

Adam Buckley original source from http://twiki.ntp.org/bin/view/Support/JavaSntpClient

Field Summary

byte	leapIndicator This is a two-bit code warning of an impending leap second to be inserted/deleted in the last minute of the current day.
byte	mode This value indicates the mode, with values defined as follows: Mode Meaning ---- ------- 0 reserved 1 symmetric active 2 symmetric passive 3 client 4 server 5 broadcast 6 reserved for NTP control message 7 reserved for private use 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.
double	originateTimestamp This is the time at which the request departed the client for the server, in seconds since 00:00 1-Jan-1900.
byte	pollInterval This value indicates the maximum interval between successive messages, in seconds to the nearest power of two.
byte	precision This value indicates the precision of the local clock, in seconds to the nearest power of two.
double	receiveTimestamp This is the time at which the request arrived at the server, in seconds since 00:00 1-Jan-1900.
byte[]	referenceIdentifier This is a 4-byte array identifying the particular reference source.
double	referenceTimestamp This is the time at which the local clock was last set or corrected, in seconds since 00:00 1-Jan-1900.
double	rootDelay This value indicates the total roundtrip delay to the primary reference source, in seconds.
double	rootDispersion This value indicates the nominal error relative to the primary reference source, in seconds.
short	stratum This value indicates the stratum level of the local clock, with values defined as follows: Stratum Meaning ---------------------------------------------- 0 unspecified or unavailable 1 primary reference (e.g., radio clock) 2-15 secondary reference (via NTP or SNTP) 16-255 reserved
double	transmitTimestamp This is the time at which the reply departed the server for the client, in seconds since 00:00 1-Jan-1900.
byte	version This value indicates the NTP/SNTP version number.

Constructor Summary

NtpMessage()
Constructs a new NtpMessage in client -> server mode, and sets the transmit timestamp to the current time.

NtpMessage(byte[] array)
Constructs a new NtpMessage from an array of bytes.

Method Summary

static double	decodeTimestamp(byte[] array, int pointer) Will read 8 bytes of a message beginning at pointer and return it as a double, according to the NTP 64-bit timestamp format.
static void	encodeTimestamp(byte[] array, int pointer, double timestamp) Encodes a timestamp in the specified position in the message
static java.lang.String	referenceIdentifierToString(byte[] ref, short stratum, byte version) Returns a string representation of a reference identifier according to the rules set out in RFC 2030.
static java.lang.String	timestampToString(double timestamp) Returns a timestamp (number of seconds since 00:00 1-Jan-1900) as a formatted date/time string.
byte[]	toByteArray() This method constructs the data bytes of a raw NTP packet.
java.lang.String	toString() Returns a string representation of a NtpMessage
static short	unsignedByteToShort(byte b) Converts an unsigned byte to a short.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

leapIndicator

public byte leapIndicator

This is a two-bit code warning of an impending leap second to be inserted/deleted in the last minute of the current day. It's values may be as follows: Value Meaning ----- ------- 0 no warning 1 last minute has 61 seconds 2 last minute has 59 seconds) 3 alarm condition (clock not synchronized)

version

public byte version

This value indicates the NTP/SNTP version number. The version number is 3 for Version 3 (IPv4 only) and 4 for Version 4 (IPv4, IPv6 and OSI). If necessary to distinguish between IPv4, IPv6 and OSI, the encapsulating context must be inspected.

mode

public byte mode

This value indicates the mode, with values defined as follows: Mode Meaning ---- ------- 0 reserved 1 symmetric active 2 symmetric passive 3 client 4 server 5 broadcast 6 reserved for NTP control message 7 reserved for private use 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 multicast mode, the server sets this field to 5 (broadcast).

stratum

public short stratum

This value indicates the stratum level of the local clock, with values defined as follows: Stratum Meaning ---------------------------------------------- 0 unspecified or unavailable 1 primary reference (e.g., radio clock) 2-15 secondary reference (via NTP or SNTP) 16-255 reserved

pollInterval

public byte pollInterval

This value indicates the maximum interval between successive messages, in seconds to the nearest power of two. The values that can appear in this field presently range from 4 (16 s) to 14 (16284 s); however, most applications use only the sub-range 6 (64 s) to 10 (1024 s).

precision

public byte precision

This value indicates the precision of the local clock, in seconds to the nearest power of two. The values that normally appear in this field range from -6 for mains-frequency clocks to -20 for microsecond clocks found in some workstations.

rootDelay

public double rootDelay

This value indicates the total roundtrip delay to the primary reference source, in seconds. Note that this variable can take on both positive and negative values, depending on the relative time and frequency offsets. The values that normally appear in this field range from negative values of a few milliseconds to positive values of several hundred milliseconds.

rootDispersion

public double rootDispersion

This value indicates the nominal error relative to the primary reference source, in seconds. The values that normally appear in this field range from 0 to several hundred milliseconds.

referenceIdentifier

public byte[] referenceIdentifier

This is a 4-byte array identifying the particular reference source. In the case of NTP Version 3 or Version 4 stratum-0 (unspecified) or stratum-1 (primary) servers, this is a four-character ASCII string, left justified and zero padded to 32 bits. In NTP Version 3 secondary servers, this is the 32-bit IPv4 address of the reference source. In NTP Version 4 secondary servers, this is the low order 32 bits of the latest transmit timestamp of the reference source. NTP primary (stratum 1) servers should set this field to a code identifying the external reference source according to the following list. 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. Code External Reference Source ---- ------------------------- LOCL uncalibrated local clock used as a primary reference for a subnet without external means of synchronization PPS atomic clock or other pulse-per-second source individually calibrated to national standards ACTS NIST dialup modem service USNO USNO modem service PTB PTB (Germany) 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 GOES Geostationary Orbit Environment Satellite

referenceTimestamp

public double referenceTimestamp

This is the time at which the local clock was last set or corrected, in seconds since 00:00 1-Jan-1900.

originateTimestamp

public double originateTimestamp

This is the time at which the request departed the client for the server, in seconds since 00:00 1-Jan-1900.

receiveTimestamp

public double receiveTimestamp

This is the time at which the request arrived at the server, in seconds since 00:00 1-Jan-1900.

transmitTimestamp

public double transmitTimestamp

This is the time at which the reply departed the server for the client, in seconds since 00:00 1-Jan-1900.

Constructor Detail

NtpMessage

public NtpMessage(byte[] array)

Constructs a new NtpMessage from an array of bytes.

NtpMessage

public NtpMessage()

Constructs a new NtpMessage in client -> server mode, and sets the transmit timestamp to the current time.

Method Detail

toByteArray

public byte[] toByteArray()

This method constructs the data bytes of a raw NTP packet.

toString

public java.lang.String toString()

Returns a string representation of a NtpMessage

Overrides:

toString in class java.lang.Object

unsignedByteToShort

public static short unsignedByteToShort(byte b)

Converts an unsigned byte to a short. By default, Java assumes that a byte is signed.

decodeTimestamp

public static double decodeTimestamp(byte[] array,
                                     int pointer)

Will read 8 bytes of a message beginning at pointer and return it as a double, according to the NTP 64-bit timestamp format.

encodeTimestamp

public static void encodeTimestamp(byte[] array,
                                   int pointer,
                                   double timestamp)

Encodes a timestamp in the specified position in the message

timestampToString

public static java.lang.String timestampToString(double timestamp)

Returns a timestamp (number of seconds since 00:00 1-Jan-1900) as a formatted date/time string.

referenceIdentifierToString

public static java.lang.String referenceIdentifierToString(byte[] ref,
                                                           short stratum,
                                                           byte version)

Returns a string representation of a reference identifier according to the rules set out in RFC 2030.