/*
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* Copyright (C) 2008 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package android.net;
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import android.annotation.UnsupportedAppUsage;
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import android.os.SystemClock;
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import android.util.Log;
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import com.android.internal.util.TrafficStatsConstants;
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import java.net.DatagramPacket;
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import java.net.DatagramSocket;
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import java.net.InetAddress;
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import java.util.Arrays;
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/**
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* {@hide}
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*
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* Simple SNTP client class for retrieving network time.
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*
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* Sample usage:
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* <pre>SntpClient client = new SntpClient();
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* if (client.requestTime("time.foo.com")) {
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* long now = client.getNtpTime() + SystemClock.elapsedRealtime() - client.getNtpTimeReference();
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* }
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* </pre>
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*/
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public class SntpClient {
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private static final String TAG = "SntpClient";
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private static final boolean DBG = true;
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private static final int REFERENCE_TIME_OFFSET = 16;
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private static final int ORIGINATE_TIME_OFFSET = 24;
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private static final int RECEIVE_TIME_OFFSET = 32;
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private static final int TRANSMIT_TIME_OFFSET = 40;
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private static final int NTP_PACKET_SIZE = 48;
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private static final int NTP_PORT = 123;
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private static final int NTP_MODE_CLIENT = 3;
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private static final int NTP_MODE_SERVER = 4;
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private static final int NTP_MODE_BROADCAST = 5;
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private static final int NTP_VERSION = 3;
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private static final int NTP_LEAP_NOSYNC = 3;
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private static final int NTP_STRATUM_DEATH = 0;
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private static final int NTP_STRATUM_MAX = 15;
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// Number of seconds between Jan 1, 1900 and Jan 1, 1970
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// 70 years plus 17 leap days
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private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L;
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// system time computed from NTP server response
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private long mNtpTime;
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// value of SystemClock.elapsedRealtime() corresponding to mNtpTime
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private long mNtpTimeReference;
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// round trip time in milliseconds
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private long mRoundTripTime;
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private static class InvalidServerReplyException extends Exception {
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public InvalidServerReplyException(String message) {
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super(message);
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}
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}
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/**
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* Sends an SNTP request to the given host and processes the response.
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*
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* @param host host name of the server.
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* @param timeout network timeout in milliseconds.
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* @param network network over which to send the request.
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* @return true if the transaction was successful.
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*/
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public boolean requestTime(String host, int timeout, Network network) {
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final Network networkForResolv = network.getPrivateDnsBypassingCopy();
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InetAddress address = null;
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try {
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address = networkForResolv.getByName(host);
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} catch (Exception e) {
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EventLogTags.writeNtpFailure(host, e.toString());
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if (DBG) Log.d(TAG, "request time failed: " + e);
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return false;
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}
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return requestTime(address, NTP_PORT, timeout, networkForResolv);
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}
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public boolean requestTime(InetAddress address, int port, int timeout, Network network) {
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DatagramSocket socket = null;
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final int oldTag = TrafficStats.getAndSetThreadStatsTag(
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TrafficStatsConstants.TAG_SYSTEM_NTP);
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try {
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socket = new DatagramSocket();
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network.bindSocket(socket);
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socket.setSoTimeout(timeout);
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byte[] buffer = new byte[NTP_PACKET_SIZE];
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DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, port);
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// set mode = 3 (client) and version = 3
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// mode is in low 3 bits of first byte
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// version is in bits 3-5 of first byte
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buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3);
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// get current time and write it to the request packet
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final long requestTime = System.currentTimeMillis();
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final long requestTicks = SystemClock.elapsedRealtime();
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writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime);
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socket.send(request);
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// read the response
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DatagramPacket response = new DatagramPacket(buffer, buffer.length);
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socket.receive(response);
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final long responseTicks = SystemClock.elapsedRealtime();
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final long responseTime = requestTime + (responseTicks - requestTicks);
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// extract the results
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final byte leap = (byte) ((buffer[0] >> 6) & 0x3);
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final byte mode = (byte) (buffer[0] & 0x7);
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final int stratum = (int) (buffer[1] & 0xff);
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final long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET);
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final long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET);
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final long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET);
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/* do sanity check according to RFC */
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// TODO: validate originateTime == requestTime.
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checkValidServerReply(leap, mode, stratum, transmitTime);
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long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime);
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// receiveTime = originateTime + transit + skew
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// responseTime = transmitTime + transit - skew
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// clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2
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// = ((originateTime + transit + skew - originateTime) +
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// (transmitTime - (transmitTime + transit - skew)))/2
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// = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2
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// = (transit + skew - transit + skew)/2
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// = (2 * skew)/2 = skew
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long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2;
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EventLogTags.writeNtpSuccess(address.toString(), roundTripTime, clockOffset);
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if (DBG) {
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Log.d(TAG, "round trip: " + roundTripTime + "ms, " +
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"clock offset: " + clockOffset + "ms");
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}
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// save our results - use the times on this side of the network latency
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// (response rather than request time)
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mNtpTime = responseTime + clockOffset;
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mNtpTimeReference = responseTicks;
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mRoundTripTime = roundTripTime;
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} catch (Exception e) {
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EventLogTags.writeNtpFailure(address.toString(), e.toString());
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if (DBG) Log.d(TAG, "request time failed: " + e);
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return false;
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} finally {
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if (socket != null) {
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socket.close();
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}
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TrafficStats.setThreadStatsTag(oldTag);
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}
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return true;
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}
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@Deprecated
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@UnsupportedAppUsage
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public boolean requestTime(String host, int timeout) {
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Log.w(TAG, "Shame on you for calling the hidden API requestTime()!");
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return false;
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}
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/**
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* Returns the time computed from the NTP transaction.
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*
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* @return time value computed from NTP server response.
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*/
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@UnsupportedAppUsage
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public long getNtpTime() {
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return mNtpTime;
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}
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/**
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* Returns the reference clock value (value of SystemClock.elapsedRealtime())
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* corresponding to the NTP time.
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*
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* @return reference clock corresponding to the NTP time.
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*/
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@UnsupportedAppUsage
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public long getNtpTimeReference() {
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return mNtpTimeReference;
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}
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/**
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* Returns the round trip time of the NTP transaction
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*
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* @return round trip time in milliseconds.
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*/
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@UnsupportedAppUsage
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public long getRoundTripTime() {
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return mRoundTripTime;
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}
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private static void checkValidServerReply(
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byte leap, byte mode, int stratum, long transmitTime)
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throws InvalidServerReplyException {
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if (leap == NTP_LEAP_NOSYNC) {
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throw new InvalidServerReplyException("unsynchronized server");
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}
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if ((mode != NTP_MODE_SERVER) && (mode != NTP_MODE_BROADCAST)) {
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throw new InvalidServerReplyException("untrusted mode: " + mode);
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}
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if ((stratum == NTP_STRATUM_DEATH) || (stratum > NTP_STRATUM_MAX)) {
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throw new InvalidServerReplyException("untrusted stratum: " + stratum);
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}
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if (transmitTime == 0) {
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throw new InvalidServerReplyException("zero transmitTime");
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}
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}
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/**
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* Reads an unsigned 32 bit big endian number from the given offset in the buffer.
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*/
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private long read32(byte[] buffer, int offset) {
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byte b0 = buffer[offset];
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byte b1 = buffer[offset+1];
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byte b2 = buffer[offset+2];
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byte b3 = buffer[offset+3];
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// convert signed bytes to unsigned values
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int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0);
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int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1);
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int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2);
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int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3);
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return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3;
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}
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/**
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* Reads the NTP time stamp at the given offset in the buffer and returns
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* it as a system time (milliseconds since January 1, 1970).
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*/
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private long readTimeStamp(byte[] buffer, int offset) {
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long seconds = read32(buffer, offset);
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long fraction = read32(buffer, offset + 4);
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// Special case: zero means zero.
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if (seconds == 0 && fraction == 0) {
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return 0;
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}
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return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L);
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}
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/**
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* Writes system time (milliseconds since January 1, 1970) as an NTP time stamp
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* at the given offset in the buffer.
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*/
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private void writeTimeStamp(byte[] buffer, int offset, long time) {
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// Special case: zero means zero.
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if (time == 0) {
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Arrays.fill(buffer, offset, offset + 8, (byte) 0x00);
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return;
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}
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long seconds = time / 1000L;
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long milliseconds = time - seconds * 1000L;
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seconds += OFFSET_1900_TO_1970;
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// write seconds in big endian format
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buffer[offset++] = (byte)(seconds >> 24);
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buffer[offset++] = (byte)(seconds >> 16);
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buffer[offset++] = (byte)(seconds >> 8);
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buffer[offset++] = (byte)(seconds >> 0);
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long fraction = milliseconds * 0x100000000L / 1000L;
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// write fraction in big endian format
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buffer[offset++] = (byte)(fraction >> 24);
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buffer[offset++] = (byte)(fraction >> 16);
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buffer[offset++] = (byte)(fraction >> 8);
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// low order bits should be random data
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buffer[offset++] = (byte)(Math.random() * 255.0);
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}
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}
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