1 /* 2 * Copyright (C) 2008 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package android.net; 18 19 import android.os.SystemClock; 20 import android.util.Log; 21 22 import java.net.DatagramPacket; 23 import java.net.DatagramSocket; 24 import java.net.InetAddress; 25 26 /** 27 * {@hide} 28 * 29 * Simple SNTP client class for retrieving network time. 30 * 31 * Sample usage: 32 * <pre>SntpClient client = new SntpClient(); 33 * if (client.requestTime("time.foo.com")) { 34 * long now = client.getNtpTime() + SystemClock.elapsedRealtime() - client.getNtpTimeReference(); 35 * } 36 * </pre> 37 */ 38 public class SntpClient 39 { 40 private static final String TAG = "SntpClient"; 41 42 private static final int REFERENCE_TIME_OFFSET = 16; 43 private static final int ORIGINATE_TIME_OFFSET = 24; 44 private static final int RECEIVE_TIME_OFFSET = 32; 45 private static final int TRANSMIT_TIME_OFFSET = 40; 46 private static final int NTP_PACKET_SIZE = 48; 47 48 private static final int NTP_PORT = 123; 49 private static final int NTP_MODE_CLIENT = 3; 50 private static final int NTP_VERSION = 3; 51 52 // Number of seconds between Jan 1, 1900 and Jan 1, 1970 53 // 70 years plus 17 leap days 54 private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L; 55 56 // system time computed from NTP server response 57 private long mNtpTime; 58 59 // value of SystemClock.elapsedRealtime() corresponding to mNtpTime 60 private long mNtpTimeReference; 61 62 // round trip time in milliseconds 63 private long mRoundTripTime; 64 65 /** 66 * Sends an SNTP request to the given host and processes the response. 67 * 68 * @param host host name of the server. 69 * @param timeout network timeout in milliseconds. 70 * @return true if the transaction was successful. 71 */ 72 public boolean requestTime(String host, int timeout) { 73 DatagramSocket socket = null; 74 try { 75 socket = new DatagramSocket(); 76 socket.setSoTimeout(timeout); 77 InetAddress address = InetAddress.getByName(host); 78 byte[] buffer = new byte[NTP_PACKET_SIZE]; 79 DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT); 80 81 // set mode = 3 (client) and version = 3 82 // mode is in low 3 bits of first byte 83 // version is in bits 3-5 of first byte 84 buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3); 85 86 // get current time and write it to the request packet 87 long requestTime = System.currentTimeMillis(); 88 long requestTicks = SystemClock.elapsedRealtime(); 89 writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime); 90 91 socket.send(request); 92 93 // read the response 94 DatagramPacket response = new DatagramPacket(buffer, buffer.length); 95 socket.receive(response); 96 long responseTicks = SystemClock.elapsedRealtime(); 97 long responseTime = requestTime + (responseTicks - requestTicks); 98 99 // extract the results 100 long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET); 101 long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET); 102 long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET); 103 long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime); 104 // receiveTime = originateTime + transit + skew 105 // responseTime = transmitTime + transit - skew 106 // clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2 107 // = ((originateTime + transit + skew - originateTime) + 108 // (transmitTime - (transmitTime + transit - skew)))/2 109 // = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2 110 // = (transit + skew - transit + skew)/2 111 // = (2 * skew)/2 = skew 112 long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2; 113 // if (false) Log.d(TAG, "round trip: " + roundTripTime + " ms"); 114 // if (false) Log.d(TAG, "clock offset: " + clockOffset + " ms"); 115 116 // save our results - use the times on this side of the network latency 117 // (response rather than request time) 118 mNtpTime = responseTime + clockOffset; 119 mNtpTimeReference = responseTicks; 120 mRoundTripTime = roundTripTime; 121 } catch (Exception e) { 122 if (false) Log.d(TAG, "request time failed: " + e); 123 return false; 124 } finally { 125 if (socket != null) { 126 socket.close(); 127 } 128 } 129 130 return true; 131 } 132 133 /** 134 * Returns the time computed from the NTP transaction. 135 * 136 * @return time value computed from NTP server response. 137 */ 138 public long getNtpTime() { 139 return mNtpTime; 140 } 141 142 /** 143 * Returns the reference clock value (value of SystemClock.elapsedRealtime()) 144 * corresponding to the NTP time. 145 * 146 * @return reference clock corresponding to the NTP time. 147 */ 148 public long getNtpTimeReference() { 149 return mNtpTimeReference; 150 } 151 152 /** 153 * Returns the round trip time of the NTP transaction 154 * 155 * @return round trip time in milliseconds. 156 */ 157 public long getRoundTripTime() { 158 return mRoundTripTime; 159 } 160 161 /** 162 * Reads an unsigned 32 bit big endian number from the given offset in the buffer. 163 */ 164 private long read32(byte[] buffer, int offset) { 165 byte b0 = buffer[offset]; 166 byte b1 = buffer[offset+1]; 167 byte b2 = buffer[offset+2]; 168 byte b3 = buffer[offset+3]; 169 170 // convert signed bytes to unsigned values 171 int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0); 172 int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1); 173 int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2); 174 int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3); 175 176 return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3; 177 } 178 179 /** 180 * Reads the NTP time stamp at the given offset in the buffer and returns 181 * it as a system time (milliseconds since January 1, 1970). 182 */ 183 private long readTimeStamp(byte[] buffer, int offset) { 184 long seconds = read32(buffer, offset); 185 long fraction = read32(buffer, offset + 4); 186 return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L); 187 } 188 189 /** 190 * Writes system time (milliseconds since January 1, 1970) as an NTP time stamp 191 * at the given offset in the buffer. 192 */ 193 private void writeTimeStamp(byte[] buffer, int offset, long time) { 194 long seconds = time / 1000L; 195 long milliseconds = time - seconds * 1000L; 196 seconds += OFFSET_1900_TO_1970; 197 198 // write seconds in big endian format 199 buffer[offset++] = (byte)(seconds >> 24); 200 buffer[offset++] = (byte)(seconds >> 16); 201 buffer[offset++] = (byte)(seconds >> 8); 202 buffer[offset++] = (byte)(seconds >> 0); 203 204 long fraction = milliseconds * 0x100000000L / 1000L; 205 // write fraction in big endian format 206 buffer[offset++] = (byte)(fraction >> 24); 207 buffer[offset++] = (byte)(fraction >> 16); 208 buffer[offset++] = (byte)(fraction >> 8); 209 // low order bits should be random data 210 buffer[offset++] = (byte)(Math.random() * 255.0); 211 } 212 } 213
