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.io.IOException; 23 import java.net.DatagramPacket; 24 import java.net.DatagramSocket; 25 import java.net.InetAddress; 26 27 /** 28 * {@hide} 29 * 30 * Simple SNTP client class for retrieving network time. 31 * 32 * Sample usage: 33 * <pre>SntpClient client = new SntpClient(); 34 * if (client.requestTime("time.foo.com")) { 35 * long now = client.getNtpTime() + SystemClock.elapsedRealtime() - client.getNtpTimeReference(); 36 * } 37 * </pre> 38 */ 39 public class SntpClient 40 { 41 private static final String TAG = "SntpClient"; 42 43 private static final int REFERENCE_TIME_OFFSET = 16; 44 private static final int ORIGINATE_TIME_OFFSET = 24; 45 private static final int RECEIVE_TIME_OFFSET = 32; 46 private static final int TRANSMIT_TIME_OFFSET = 40; 47 private static final int NTP_PACKET_SIZE = 48; 48 49 private static final int NTP_PORT = 123; 50 private static final int NTP_MODE_CLIENT = 3; 51 private static final int NTP_VERSION = 3; 52 53 // Number of seconds between Jan 1, 1900 and Jan 1, 1970 54 // 70 years plus 17 leap days 55 private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L; 56 57 // system time computed from NTP server response 58 private long mNtpTime; 59 60 // value of SystemClock.elapsedRealtime() corresponding to mNtpTime 61 private long mNtpTimeReference; 62 63 // round trip time in milliseconds 64 private long mRoundTripTime; 65 66 /** 67 * Sends an SNTP request to the given host and processes the response. 68 * 69 * @param host host name of the server. 70 * @param timeout network timeout in milliseconds. 71 * @return true if the transaction was successful. 72 */ 73 public boolean requestTime(String host, int timeout) { 74 DatagramSocket socket = null; 75 try { 76 socket = new DatagramSocket(); 77 socket.setSoTimeout(timeout); 78 InetAddress address = InetAddress.getByName(host); 79 byte[] buffer = new byte[NTP_PACKET_SIZE]; 80 DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT); 81 82 // set mode = 3 (client) and version = 3 83 // mode is in low 3 bits of first byte 84 // version is in bits 3-5 of first byte 85 buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3); 86 87 // get current time and write it to the request packet 88 long requestTime = System.currentTimeMillis(); 89 long requestTicks = SystemClock.elapsedRealtime(); 90 writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime); 91 92 socket.send(request); 93 94 // read the response 95 DatagramPacket response = new DatagramPacket(buffer, buffer.length); 96 socket.receive(response); 97 long responseTicks = SystemClock.elapsedRealtime(); 98 long responseTime = requestTime + (responseTicks - requestTicks); 99 100 // extract the results 101 long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET); 102 long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET); 103 long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET); 104 long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime); 105 // receiveTime = originateTime + transit + skew 106 // responseTime = transmitTime + transit - skew 107 // clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2 108 // = ((originateTime + transit + skew - originateTime) + 109 // (transmitTime - (transmitTime + transit - skew)))/2 110 // = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2 111 // = (transit + skew - transit + skew)/2 112 // = (2 * skew)/2 = skew 113 long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2; 114 // if (false) Log.d(TAG, "round trip: " + roundTripTime + " ms"); 115 // if (false) Log.d(TAG, "clock offset: " + clockOffset + " ms"); 116 117 // save our results - use the times on this side of the network latency 118 // (response rather than request time) 119 mNtpTime = responseTime + clockOffset; 120 mNtpTimeReference = responseTicks; 121 mRoundTripTime = roundTripTime; 122 } catch (Exception e) { 123 if (false) Log.d(TAG, "request time failed: " + e); 124 return false; 125 } finally { 126 if (socket != null) { 127 socket.close(); 128 } 129 } 130 131 return true; 132 } 133 134 /** 135 * Returns the time computed from the NTP transaction. 136 * 137 * @return time value computed from NTP server response. 138 */ 139 public long getNtpTime() { 140 return mNtpTime; 141 } 142 143 /** 144 * Returns the reference clock value (value of SystemClock.elapsedRealtime()) 145 * corresponding to the NTP time. 146 * 147 * @return reference clock corresponding to the NTP time. 148 */ 149 public long getNtpTimeReference() { 150 return mNtpTimeReference; 151 } 152 153 /** 154 * Returns the round trip time of the NTP transaction 155 * 156 * @return round trip time in milliseconds. 157 */ 158 public long getRoundTripTime() { 159 return mRoundTripTime; 160 } 161 162 /** 163 * Reads an unsigned 32 bit big endian number from the given offset in the buffer. 164 */ 165 private long read32(byte[] buffer, int offset) { 166 byte b0 = buffer[offset]; 167 byte b1 = buffer[offset+1]; 168 byte b2 = buffer[offset+2]; 169 byte b3 = buffer[offset+3]; 170 171 // convert signed bytes to unsigned values 172 int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0); 173 int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1); 174 int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2); 175 int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3); 176 177 return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3; 178 } 179 180 /** 181 * Reads the NTP time stamp at the given offset in the buffer and returns 182 * it as a system time (milliseconds since January 1, 1970). 183 */ 184 private long readTimeStamp(byte[] buffer, int offset) { 185 long seconds = read32(buffer, offset); 186 long fraction = read32(buffer, offset + 4); 187 return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L); 188 } 189 190 /** 191 * Writes system time (milliseconds since January 1, 1970) as an NTP time stamp 192 * at the given offset in the buffer. 193 */ 194 private void writeTimeStamp(byte[] buffer, int offset, long time) { 195 long seconds = time / 1000L; 196 long milliseconds = time - seconds * 1000L; 197 seconds += OFFSET_1900_TO_1970; 198 199 // write seconds in big endian format 200 buffer[offset++] = (byte)(seconds >> 24); 201 buffer[offset++] = (byte)(seconds >> 16); 202 buffer[offset++] = (byte)(seconds >> 8); 203 buffer[offset++] = (byte)(seconds >> 0); 204 205 long fraction = milliseconds * 0x100000000L / 1000L; 206 // write fraction in big endian format 207 buffer[offset++] = (byte)(fraction >> 24); 208 buffer[offset++] = (byte)(fraction >> 16); 209 buffer[offset++] = (byte)(fraction >> 8); 210 // low order bits should be random data 211 buffer[offset++] = (byte)(Math.random() * 255.0); 212 } 213 } 214
