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.Config; 21 import android.util.Log; 22 23 import java.io.IOException; 24 import java.net.DatagramPacket; 25 import java.net.DatagramSocket; 26 import java.net.InetAddress; 27 28 /** 29 * {@hide} 30 * 31 * Simple SNTP client class for retrieving network time. 32 * 33 * Sample usage: 34 * <pre>SntpClient client = new SntpClient(); 35 * if (client.requestTime("time.foo.com")) { 36 * long now = client.getNtpTime() + SystemClock.elapsedRealtime() - client.getNtpTimeReference(); 37 * } 38 * </pre> 39 */ 40 public class SntpClient 41 { 42 private static final String TAG = "SntpClient"; 43 44 private static final int REFERENCE_TIME_OFFSET = 16; 45 private static final int ORIGINATE_TIME_OFFSET = 24; 46 private static final int RECEIVE_TIME_OFFSET = 32; 47 private static final int TRANSMIT_TIME_OFFSET = 40; 48 private static final int NTP_PACKET_SIZE = 48; 49 50 private static final int NTP_PORT = 123; 51 private static final int NTP_MODE_CLIENT = 3; 52 private static final int NTP_VERSION = 3; 53 54 // Number of seconds between Jan 1, 1900 and Jan 1, 1970 55 // 70 years plus 17 leap days 56 private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L; 57 58 // system time computed from NTP server response 59 private long mNtpTime; 60 61 // value of SystemClock.elapsedRealtime() corresponding to mNtpTime 62 private long mNtpTimeReference; 63 64 // round trip time in milliseconds 65 private long mRoundTripTime; 66 67 /** 68 * Sends an SNTP request to the given host and processes the response. 69 * 70 * @param host host name of the server. 71 * @param timeout network timeout in milliseconds. 72 * @return true if the transaction was successful. 73 */ 74 public boolean requestTime(String host, int timeout) { 75 DatagramSocket socket = null; 76 try { 77 socket = new DatagramSocket(); 78 socket.setSoTimeout(timeout); 79 InetAddress address = InetAddress.getByName(host); 80 byte[] buffer = new byte[NTP_PACKET_SIZE]; 81 DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT); 82 83 // set mode = 3 (client) and version = 3 84 // mode is in low 3 bits of first byte 85 // version is in bits 3-5 of first byte 86 buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3); 87 88 // get current time and write it to the request packet 89 long requestTime = System.currentTimeMillis(); 90 long requestTicks = SystemClock.elapsedRealtime(); 91 writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime); 92 93 socket.send(request); 94 95 // read the response 96 DatagramPacket response = new DatagramPacket(buffer, buffer.length); 97 socket.receive(response); 98 long responseTicks = SystemClock.elapsedRealtime(); 99 long responseTime = requestTime + (responseTicks - requestTicks); 100 101 // extract the results 102 long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET); 103 long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET); 104 long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET); 105 long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime); 106 // receiveTime = originateTime + transit + skew 107 // responseTime = transmitTime + transit - skew 108 // clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2 109 // = ((originateTime + transit + skew - originateTime) + 110 // (transmitTime - (transmitTime + transit - skew)))/2 111 // = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2 112 // = (transit + skew - transit + skew)/2 113 // = (2 * skew)/2 = skew 114 long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2; 115 // if (Config.LOGD) Log.d(TAG, "round trip: " + roundTripTime + " ms"); 116 // if (Config.LOGD) Log.d(TAG, "clock offset: " + clockOffset + " ms"); 117 118 // save our results - use the times on this side of the network latency 119 // (response rather than request time) 120 mNtpTime = responseTime + clockOffset; 121 mNtpTimeReference = responseTicks; 122 mRoundTripTime = roundTripTime; 123 } catch (Exception e) { 124 if (Config.LOGD) Log.d(TAG, "request time failed: " + e); 125 return false; 126 } finally { 127 if (socket != null) { 128 socket.close(); 129 } 130 } 131 132 return true; 133 } 134 135 /** 136 * Returns the time computed from the NTP transaction. 137 * 138 * @return time value computed from NTP server response. 139 */ 140 public long getNtpTime() { 141 return mNtpTime; 142 } 143 144 /** 145 * Returns the reference clock value (value of SystemClock.elapsedRealtime()) 146 * corresponding to the NTP time. 147 * 148 * @return reference clock corresponding to the NTP time. 149 */ 150 public long getNtpTimeReference() { 151 return mNtpTimeReference; 152 } 153 154 /** 155 * Returns the round trip time of the NTP transaction 156 * 157 * @return round trip time in milliseconds. 158 */ 159 public long getRoundTripTime() { 160 return mRoundTripTime; 161 } 162 163 /** 164 * Reads an unsigned 32 bit big endian number from the given offset in the buffer. 165 */ 166 private long read32(byte[] buffer, int offset) { 167 byte b0 = buffer[offset]; 168 byte b1 = buffer[offset+1]; 169 byte b2 = buffer[offset+2]; 170 byte b3 = buffer[offset+3]; 171 172 // convert signed bytes to unsigned values 173 int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0); 174 int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1); 175 int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2); 176 int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3); 177 178 return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3; 179 } 180 181 /** 182 * Reads the NTP time stamp at the given offset in the buffer and returns 183 * it as a system time (milliseconds since January 1, 1970). 184 */ 185 private long readTimeStamp(byte[] buffer, int offset) { 186 long seconds = read32(buffer, offset); 187 long fraction = read32(buffer, offset + 4); 188 return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L); 189 } 190 191 /** 192 * Writes system time (milliseconds since January 1, 1970) as an NTP time stamp 193 * at the given offset in the buffer. 194 */ 195 private void writeTimeStamp(byte[] buffer, int offset, long time) { 196 long seconds = time / 1000L; 197 long milliseconds = time - seconds * 1000L; 198 seconds += OFFSET_1900_TO_1970; 199 200 // write seconds in big endian format 201 buffer[offset++] = (byte)(seconds >> 24); 202 buffer[offset++] = (byte)(seconds >> 16); 203 buffer[offset++] = (byte)(seconds >> 8); 204 buffer[offset++] = (byte)(seconds >> 0); 205 206 long fraction = milliseconds * 0x100000000L / 1000L; 207 // write fraction in big endian format 208 buffer[offset++] = (byte)(fraction >> 24); 209 buffer[offset++] = (byte)(fraction >> 16); 210 buffer[offset++] = (byte)(fraction >> 8); 211 // low order bits should be random data 212 buffer[offset++] = (byte)(Math.random() * 255.0); 213 } 214 } 215
