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 import java.util.Arrays; 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 private static final String TAG = "SntpClient"; 41 private static final boolean DBG = true; 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_MODE_SERVER = 4; 52 private static final int NTP_MODE_BROADCAST = 5; 53 private static final int NTP_VERSION = 3; 54 55 private static final int NTP_LEAP_NOSYNC = 3; 56 private static final int NTP_STRATUM_DEATH = 0; 57 private static final int NTP_STRATUM_MAX = 15; 58 59 // Number of seconds between Jan 1, 1900 and Jan 1, 1970 60 // 70 years plus 17 leap days 61 private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L; 62 63 // system time computed from NTP server response 64 private long mNtpTime; 65 66 // value of SystemClock.elapsedRealtime() corresponding to mNtpTime 67 private long mNtpTimeReference; 68 69 // round trip time in milliseconds 70 private long mRoundTripTime; 71 72 private static class InvalidServerReplyException extends Exception { 73 public InvalidServerReplyException(String message) { 74 super(message); 75 } 76 } 77 78 /** 79 * Sends an SNTP request to the given host and processes the response. 80 * 81 * @param host host name of the server. 82 * @param timeout network timeout in milliseconds. 83 * @return true if the transaction was successful. 84 */ 85 public boolean requestTime(String host, int timeout) { 86 InetAddress address = null; 87 try { 88 address = InetAddress.getByName(host); 89 } catch (Exception e) { 90 EventLogTags.writeNtpFailure(host, e.toString()); 91 if (DBG) Log.d(TAG, "request time failed: " + e); 92 return false; 93 } 94 return requestTime(address, NTP_PORT, timeout); 95 } 96 97 public boolean requestTime(InetAddress address, int port, int timeout) { 98 DatagramSocket socket = null; 99 final int oldTag = TrafficStats.getAndSetThreadStatsTag(TrafficStats.TAG_SYSTEM_NTP); 100 try { 101 socket = new DatagramSocket(); 102 socket.setSoTimeout(timeout); 103 byte[] buffer = new byte[NTP_PACKET_SIZE]; 104 DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, port); 105 106 // set mode = 3 (client) and version = 3 107 // mode is in low 3 bits of first byte 108 // version is in bits 3-5 of first byte 109 buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3); 110 111 // get current time and write it to the request packet 112 final long requestTime = System.currentTimeMillis(); 113 final long requestTicks = SystemClock.elapsedRealtime(); 114 writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime); 115 116 socket.send(request); 117 118 // read the response 119 DatagramPacket response = new DatagramPacket(buffer, buffer.length); 120 socket.receive(response); 121 final long responseTicks = SystemClock.elapsedRealtime(); 122 final long responseTime = requestTime + (responseTicks - requestTicks); 123 124 // extract the results 125 final byte leap = (byte) ((buffer[0] >> 6) & 0x3); 126 final byte mode = (byte) (buffer[0] & 0x7); 127 final int stratum = (int) (buffer[1] & 0xff); 128 final long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET); 129 final long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET); 130 final long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET); 131 132 /* do sanity check according to RFC */ 133 // TODO: validate originateTime == requestTime. 134 checkValidServerReply(leap, mode, stratum, transmitTime); 135 136 long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime); 137 // receiveTime = originateTime + transit + skew 138 // responseTime = transmitTime + transit - skew 139 // clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2 140 // = ((originateTime + transit + skew - originateTime) + 141 // (transmitTime - (transmitTime + transit - skew)))/2 142 // = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2 143 // = (transit + skew - transit + skew)/2 144 // = (2 * skew)/2 = skew 145 long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2; 146 EventLogTags.writeNtpSuccess(address.toString(), roundTripTime, clockOffset); 147 if (DBG) { 148 Log.d(TAG, "round trip: " + roundTripTime + "ms, " + 149 "clock offset: " + clockOffset + "ms"); 150 } 151 152 // save our results - use the times on this side of the network latency 153 // (response rather than request time) 154 mNtpTime = responseTime + clockOffset; 155 mNtpTimeReference = responseTicks; 156 mRoundTripTime = roundTripTime; 157 } catch (Exception e) { 158 EventLogTags.writeNtpFailure(address.toString(), e.toString()); 159 if (DBG) Log.d(TAG, "request time failed: " + e); 160 return false; 161 } finally { 162 if (socket != null) { 163 socket.close(); 164 } 165 TrafficStats.setThreadStatsTag(oldTag); 166 } 167 168 return true; 169 } 170 171 /** 172 * Returns the time computed from the NTP transaction. 173 * 174 * @return time value computed from NTP server response. 175 */ 176 public long getNtpTime() { 177 return mNtpTime; 178 } 179 180 /** 181 * Returns the reference clock value (value of SystemClock.elapsedRealtime()) 182 * corresponding to the NTP time. 183 * 184 * @return reference clock corresponding to the NTP time. 185 */ 186 public long getNtpTimeReference() { 187 return mNtpTimeReference; 188 } 189 190 /** 191 * Returns the round trip time of the NTP transaction 192 * 193 * @return round trip time in milliseconds. 194 */ 195 public long getRoundTripTime() { 196 return mRoundTripTime; 197 } 198 199 private static void checkValidServerReply( 200 byte leap, byte mode, int stratum, long transmitTime) 201 throws InvalidServerReplyException { 202 if (leap == NTP_LEAP_NOSYNC) { 203 throw new InvalidServerReplyException("unsynchronized server"); 204 } 205 if ((mode != NTP_MODE_SERVER) && (mode != NTP_MODE_BROADCAST)) { 206 throw new InvalidServerReplyException("untrusted mode: " + mode); 207 } 208 if ((stratum == NTP_STRATUM_DEATH) || (stratum > NTP_STRATUM_MAX)) { 209 throw new InvalidServerReplyException("untrusted stratum: " + stratum); 210 } 211 if (transmitTime == 0) { 212 throw new InvalidServerReplyException("zero transmitTime"); 213 } 214 } 215 216 /** 217 * Reads an unsigned 32 bit big endian number from the given offset in the buffer. 218 */ 219 private long read32(byte[] buffer, int offset) { 220 byte b0 = buffer[offset]; 221 byte b1 = buffer[offset+1]; 222 byte b2 = buffer[offset+2]; 223 byte b3 = buffer[offset+3]; 224 225 // convert signed bytes to unsigned values 226 int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0); 227 int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1); 228 int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2); 229 int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3); 230 231 return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3; 232 } 233 234 /** 235 * Reads the NTP time stamp at the given offset in the buffer and returns 236 * it as a system time (milliseconds since January 1, 1970). 237 */ 238 private long readTimeStamp(byte[] buffer, int offset) { 239 long seconds = read32(buffer, offset); 240 long fraction = read32(buffer, offset + 4); 241 // Special case: zero means zero. 242 if (seconds == 0 && fraction == 0) { 243 return 0; 244 } 245 return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L); 246 } 247 248 /** 249 * Writes system time (milliseconds since January 1, 1970) as an NTP time stamp 250 * at the given offset in the buffer. 251 */ 252 private void writeTimeStamp(byte[] buffer, int offset, long time) { 253 // Special case: zero means zero. 254 if (time == 0) { 255 Arrays.fill(buffer, offset, offset + 8, (byte) 0x00); 256 return; 257 } 258 259 long seconds = time / 1000L; 260 long milliseconds = time - seconds * 1000L; 261 seconds += OFFSET_1900_TO_1970; 262 263 // write seconds in big endian format 264 buffer[offset++] = (byte)(seconds >> 24); 265 buffer[offset++] = (byte)(seconds >> 16); 266 buffer[offset++] = (byte)(seconds >> 8); 267 buffer[offset++] = (byte)(seconds >> 0); 268 269 long fraction = milliseconds * 0x100000000L / 1000L; 270 // write fraction in big endian format 271 buffer[offset++] = (byte)(fraction >> 24); 272 buffer[offset++] = (byte)(fraction >> 16); 273 buffer[offset++] = (byte)(fraction >> 8); 274 // low order bits should be random data 275 buffer[offset++] = (byte)(Math.random() * 255.0); 276 } 277 } 278
