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 * @param network network over which to send the request. 84 * @return true if the transaction was successful. 85 */ 86 public boolean requestTime(String host, int timeout, Network network) { 87 // This flag only affects DNS resolution and not other socket semantics, 88 // therefore it's safe to set unilaterally rather than take more 89 // defensive measures like making a copy. 90 network.setPrivateDnsBypass(true); 91 InetAddress address = null; 92 try { 93 address = network.getByName(host); 94 } catch (Exception e) { 95 EventLogTags.writeNtpFailure(host, e.toString()); 96 if (DBG) Log.d(TAG, "request time failed: " + e); 97 return false; 98 } 99 return requestTime(address, NTP_PORT, timeout, network); 100 } 101 102 public boolean requestTime(InetAddress address, int port, int timeout, Network network) { 103 DatagramSocket socket = null; 104 final int oldTag = TrafficStats.getAndSetThreadStatsTag(TrafficStats.TAG_SYSTEM_NTP); 105 try { 106 socket = new DatagramSocket(); 107 network.bindSocket(socket); 108 socket.setSoTimeout(timeout); 109 byte[] buffer = new byte[NTP_PACKET_SIZE]; 110 DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, port); 111 112 // set mode = 3 (client) and version = 3 113 // mode is in low 3 bits of first byte 114 // version is in bits 3-5 of first byte 115 buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3); 116 117 // get current time and write it to the request packet 118 final long requestTime = System.currentTimeMillis(); 119 final long requestTicks = SystemClock.elapsedRealtime(); 120 writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime); 121 122 socket.send(request); 123 124 // read the response 125 DatagramPacket response = new DatagramPacket(buffer, buffer.length); 126 socket.receive(response); 127 final long responseTicks = SystemClock.elapsedRealtime(); 128 final long responseTime = requestTime + (responseTicks - requestTicks); 129 130 // extract the results 131 final byte leap = (byte) ((buffer[0] >> 6) & 0x3); 132 final byte mode = (byte) (buffer[0] & 0x7); 133 final int stratum = (int) (buffer[1] & 0xff); 134 final long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET); 135 final long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET); 136 final long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET); 137 138 /* do sanity check according to RFC */ 139 // TODO: validate originateTime == requestTime. 140 checkValidServerReply(leap, mode, stratum, transmitTime); 141 142 long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime); 143 // receiveTime = originateTime + transit + skew 144 // responseTime = transmitTime + transit - skew 145 // clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2 146 // = ((originateTime + transit + skew - originateTime) + 147 // (transmitTime - (transmitTime + transit - skew)))/2 148 // = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2 149 // = (transit + skew - transit + skew)/2 150 // = (2 * skew)/2 = skew 151 long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2; 152 EventLogTags.writeNtpSuccess(address.toString(), roundTripTime, clockOffset); 153 if (DBG) { 154 Log.d(TAG, "round trip: " + roundTripTime + "ms, " + 155 "clock offset: " + clockOffset + "ms"); 156 } 157 158 // save our results - use the times on this side of the network latency 159 // (response rather than request time) 160 mNtpTime = responseTime + clockOffset; 161 mNtpTimeReference = responseTicks; 162 mRoundTripTime = roundTripTime; 163 } catch (Exception e) { 164 EventLogTags.writeNtpFailure(address.toString(), e.toString()); 165 if (DBG) Log.d(TAG, "request time failed: " + e); 166 return false; 167 } finally { 168 if (socket != null) { 169 socket.close(); 170 } 171 TrafficStats.setThreadStatsTag(oldTag); 172 } 173 174 return true; 175 } 176 177 @Deprecated 178 public boolean requestTime(String host, int timeout) { 179 Log.w(TAG, "Shame on you for calling the hidden API requestTime()!"); 180 return false; 181 } 182 183 /** 184 * Returns the time computed from the NTP transaction. 185 * 186 * @return time value computed from NTP server response. 187 */ 188 public long getNtpTime() { 189 return mNtpTime; 190 } 191 192 /** 193 * Returns the reference clock value (value of SystemClock.elapsedRealtime()) 194 * corresponding to the NTP time. 195 * 196 * @return reference clock corresponding to the NTP time. 197 */ 198 public long getNtpTimeReference() { 199 return mNtpTimeReference; 200 } 201 202 /** 203 * Returns the round trip time of the NTP transaction 204 * 205 * @return round trip time in milliseconds. 206 */ 207 public long getRoundTripTime() { 208 return mRoundTripTime; 209 } 210 211 private static void checkValidServerReply( 212 byte leap, byte mode, int stratum, long transmitTime) 213 throws InvalidServerReplyException { 214 if (leap == NTP_LEAP_NOSYNC) { 215 throw new InvalidServerReplyException("unsynchronized server"); 216 } 217 if ((mode != NTP_MODE_SERVER) && (mode != NTP_MODE_BROADCAST)) { 218 throw new InvalidServerReplyException("untrusted mode: " + mode); 219 } 220 if ((stratum == NTP_STRATUM_DEATH) || (stratum > NTP_STRATUM_MAX)) { 221 throw new InvalidServerReplyException("untrusted stratum: " + stratum); 222 } 223 if (transmitTime == 0) { 224 throw new InvalidServerReplyException("zero transmitTime"); 225 } 226 } 227 228 /** 229 * Reads an unsigned 32 bit big endian number from the given offset in the buffer. 230 */ 231 private long read32(byte[] buffer, int offset) { 232 byte b0 = buffer[offset]; 233 byte b1 = buffer[offset+1]; 234 byte b2 = buffer[offset+2]; 235 byte b3 = buffer[offset+3]; 236 237 // convert signed bytes to unsigned values 238 int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0); 239 int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1); 240 int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2); 241 int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3); 242 243 return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3; 244 } 245 246 /** 247 * Reads the NTP time stamp at the given offset in the buffer and returns 248 * it as a system time (milliseconds since January 1, 1970). 249 */ 250 private long readTimeStamp(byte[] buffer, int offset) { 251 long seconds = read32(buffer, offset); 252 long fraction = read32(buffer, offset + 4); 253 // Special case: zero means zero. 254 if (seconds == 0 && fraction == 0) { 255 return 0; 256 } 257 return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L); 258 } 259 260 /** 261 * Writes system time (milliseconds since January 1, 1970) as an NTP time stamp 262 * at the given offset in the buffer. 263 */ 264 private void writeTimeStamp(byte[] buffer, int offset, long time) { 265 // Special case: zero means zero. 266 if (time == 0) { 267 Arrays.fill(buffer, offset, offset + 8, (byte) 0x00); 268 return; 269 } 270 271 long seconds = time / 1000L; 272 long milliseconds = time - seconds * 1000L; 273 seconds += OFFSET_1900_TO_1970; 274 275 // write seconds in big endian format 276 buffer[offset++] = (byte)(seconds >> 24); 277 buffer[offset++] = (byte)(seconds >> 16); 278 buffer[offset++] = (byte)(seconds >> 8); 279 buffer[offset++] = (byte)(seconds >> 0); 280 281 long fraction = milliseconds * 0x100000000L / 1000L; 282 // write fraction in big endian format 283 buffer[offset++] = (byte)(fraction >> 24); 284 buffer[offset++] = (byte)(fraction >> 16); 285 buffer[offset++] = (byte)(fraction >> 8); 286 // low order bits should be random data 287 buffer[offset++] = (byte)(Math.random() * 255.0); 288 } 289 } 290
