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.compat.annotation.UnsupportedAppUsage; 20 import android.os.SystemClock; 21 import android.util.Log; 22 23 import com.android.internal.util.TrafficStatsConstants; 24 25 import java.net.DatagramPacket; 26 import java.net.DatagramSocket; 27 import java.net.InetAddress; 28 import java.net.UnknownHostException; 29 import java.util.Arrays; 30 31 /** 32 * {@hide} 33 * 34 * Simple SNTP client class for retrieving network time. 35 * 36 * Sample usage: 37 * <pre>SntpClient client = new SntpClient(); 38 * if (client.requestTime("time.foo.com")) { 39 * long now = client.getNtpTime() + SystemClock.elapsedRealtime() - client.getNtpTimeReference(); 40 * } 41 * </pre> 42 */ 43 public class SntpClient { 44 private static final String TAG = "SntpClient"; 45 private static final boolean DBG = true; 46 47 private static final int REFERENCE_TIME_OFFSET = 16; 48 private static final int ORIGINATE_TIME_OFFSET = 24; 49 private static final int RECEIVE_TIME_OFFSET = 32; 50 private static final int TRANSMIT_TIME_OFFSET = 40; 51 private static final int NTP_PACKET_SIZE = 48; 52 53 private static final int NTP_PORT = 123; 54 private static final int NTP_MODE_CLIENT = 3; 55 private static final int NTP_MODE_SERVER = 4; 56 private static final int NTP_MODE_BROADCAST = 5; 57 private static final int NTP_VERSION = 3; 58 59 private static final int NTP_LEAP_NOSYNC = 3; 60 private static final int NTP_STRATUM_DEATH = 0; 61 private static final int NTP_STRATUM_MAX = 15; 62 63 // Number of seconds between Jan 1, 1900 and Jan 1, 1970 64 // 70 years plus 17 leap days 65 private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L; 66 67 // system time computed from NTP server response 68 private long mNtpTime; 69 70 // value of SystemClock.elapsedRealtime() corresponding to mNtpTime 71 private long mNtpTimeReference; 72 73 // round trip time in milliseconds 74 private long mRoundTripTime; 75 76 private static class InvalidServerReplyException extends Exception { InvalidServerReplyException(String message)77 public InvalidServerReplyException(String message) { 78 super(message); 79 } 80 } 81 82 @UnsupportedAppUsage SntpClient()83 public SntpClient() { 84 } 85 86 /** 87 * Sends an SNTP request to the given host and processes the response. 88 * 89 * @param host host name of the server. 90 * @param timeout network timeout in milliseconds. the timeout doesn't include the DNS lookup 91 * time, and it applies to each individual query to the resolved addresses of 92 * the NTP server. 93 * @param network network over which to send the request. 94 * @return true if the transaction was successful. 95 */ requestTime(String host, int timeout, Network network)96 public boolean requestTime(String host, int timeout, Network network) { 97 final Network networkForResolv = network.getPrivateDnsBypassingCopy(); 98 try { 99 final InetAddress[] addresses = networkForResolv.getAllByName(host); 100 for (int i = 0; i < addresses.length; i++) { 101 if (requestTime(addresses[i], NTP_PORT, timeout, networkForResolv)) return true; 102 } 103 } catch (UnknownHostException e) { 104 Log.w(TAG, "Unknown host: " + host); 105 EventLogTags.writeNtpFailure(host, e.toString()); 106 } 107 108 if (DBG) Log.d(TAG, "request time failed"); 109 return false; 110 } 111 requestTime(InetAddress address, int port, int timeout, Network network)112 public boolean requestTime(InetAddress address, int port, int timeout, Network network) { 113 DatagramSocket socket = null; 114 final int oldTag = TrafficStats.getAndSetThreadStatsTag( 115 TrafficStatsConstants.TAG_SYSTEM_NTP); 116 try { 117 socket = new DatagramSocket(); 118 network.bindSocket(socket); 119 socket.setSoTimeout(timeout); 120 byte[] buffer = new byte[NTP_PACKET_SIZE]; 121 DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, port); 122 123 // set mode = 3 (client) and version = 3 124 // mode is in low 3 bits of first byte 125 // version is in bits 3-5 of first byte 126 buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3); 127 128 // get current time and write it to the request packet 129 final long requestTime = System.currentTimeMillis(); 130 final long requestTicks = SystemClock.elapsedRealtime(); 131 writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime); 132 133 socket.send(request); 134 135 // read the response 136 DatagramPacket response = new DatagramPacket(buffer, buffer.length); 137 socket.receive(response); 138 final long responseTicks = SystemClock.elapsedRealtime(); 139 final long responseTime = requestTime + (responseTicks - requestTicks); 140 141 // extract the results 142 final byte leap = (byte) ((buffer[0] >> 6) & 0x3); 143 final byte mode = (byte) (buffer[0] & 0x7); 144 final int stratum = (int) (buffer[1] & 0xff); 145 final long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET); 146 final long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET); 147 final long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET); 148 final long referenceTime = readTimeStamp(buffer, REFERENCE_TIME_OFFSET); 149 150 /* Do validation according to RFC */ 151 // TODO: validate originateTime == requestTime. 152 checkValidServerReply(leap, mode, stratum, transmitTime, referenceTime); 153 154 long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime); 155 // receiveTime = originateTime + transit + skew 156 // responseTime = transmitTime + transit - skew 157 // clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2 158 // = ((originateTime + transit + skew - originateTime) + 159 // (transmitTime - (transmitTime + transit - skew)))/2 160 // = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2 161 // = (transit + skew - transit + skew)/2 162 // = (2 * skew)/2 = skew 163 long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2; 164 EventLogTags.writeNtpSuccess(address.toString(), roundTripTime, clockOffset); 165 if (DBG) { 166 Log.d(TAG, "round trip: " + roundTripTime + "ms, " + 167 "clock offset: " + clockOffset + "ms"); 168 } 169 170 // save our results - use the times on this side of the network latency 171 // (response rather than request time) 172 mNtpTime = responseTime + clockOffset; 173 mNtpTimeReference = responseTicks; 174 mRoundTripTime = roundTripTime; 175 } catch (Exception e) { 176 EventLogTags.writeNtpFailure(address.toString(), e.toString()); 177 if (DBG) Log.d(TAG, "request time failed: " + e); 178 return false; 179 } finally { 180 if (socket != null) { 181 socket.close(); 182 } 183 TrafficStats.setThreadStatsTag(oldTag); 184 } 185 186 return true; 187 } 188 189 @Deprecated 190 @UnsupportedAppUsage requestTime(String host, int timeout)191 public boolean requestTime(String host, int timeout) { 192 Log.w(TAG, "Shame on you for calling the hidden API requestTime()!"); 193 return false; 194 } 195 196 /** 197 * Returns the time computed from the NTP transaction. 198 * 199 * @return time value computed from NTP server response. 200 */ 201 @UnsupportedAppUsage getNtpTime()202 public long getNtpTime() { 203 return mNtpTime; 204 } 205 206 /** 207 * Returns the reference clock value (value of SystemClock.elapsedRealtime()) 208 * corresponding to the NTP time. 209 * 210 * @return reference clock corresponding to the NTP time. 211 */ 212 @UnsupportedAppUsage getNtpTimeReference()213 public long getNtpTimeReference() { 214 return mNtpTimeReference; 215 } 216 217 /** 218 * Returns the round trip time of the NTP transaction 219 * 220 * @return round trip time in milliseconds. 221 */ 222 @UnsupportedAppUsage getRoundTripTime()223 public long getRoundTripTime() { 224 return mRoundTripTime; 225 } 226 checkValidServerReply( byte leap, byte mode, int stratum, long transmitTime, long referenceTime)227 private static void checkValidServerReply( 228 byte leap, byte mode, int stratum, long transmitTime, long referenceTime) 229 throws InvalidServerReplyException { 230 if (leap == NTP_LEAP_NOSYNC) { 231 throw new InvalidServerReplyException("unsynchronized server"); 232 } 233 if ((mode != NTP_MODE_SERVER) && (mode != NTP_MODE_BROADCAST)) { 234 throw new InvalidServerReplyException("untrusted mode: " + mode); 235 } 236 if ((stratum == NTP_STRATUM_DEATH) || (stratum > NTP_STRATUM_MAX)) { 237 throw new InvalidServerReplyException("untrusted stratum: " + stratum); 238 } 239 if (transmitTime == 0) { 240 throw new InvalidServerReplyException("zero transmitTime"); 241 } 242 if (referenceTime == 0) { 243 throw new InvalidServerReplyException("zero reference timestamp"); 244 } 245 } 246 247 /** 248 * Reads an unsigned 32 bit big endian number from the given offset in the buffer. 249 */ read32(byte[] buffer, int offset)250 private long read32(byte[] buffer, int offset) { 251 byte b0 = buffer[offset]; 252 byte b1 = buffer[offset+1]; 253 byte b2 = buffer[offset+2]; 254 byte b3 = buffer[offset+3]; 255 256 // convert signed bytes to unsigned values 257 int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0); 258 int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1); 259 int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2); 260 int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3); 261 262 return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3; 263 } 264 265 /** 266 * Reads the NTP time stamp at the given offset in the buffer and returns 267 * it as a system time (milliseconds since January 1, 1970). 268 */ readTimeStamp(byte[] buffer, int offset)269 private long readTimeStamp(byte[] buffer, int offset) { 270 long seconds = read32(buffer, offset); 271 long fraction = read32(buffer, offset + 4); 272 // Special case: zero means zero. 273 if (seconds == 0 && fraction == 0) { 274 return 0; 275 } 276 return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L); 277 } 278 279 /** 280 * Writes system time (milliseconds since January 1, 1970) as an NTP time stamp 281 * at the given offset in the buffer. 282 */ writeTimeStamp(byte[] buffer, int offset, long time)283 private void writeTimeStamp(byte[] buffer, int offset, long time) { 284 // Special case: zero means zero. 285 if (time == 0) { 286 Arrays.fill(buffer, offset, offset + 8, (byte) 0x00); 287 return; 288 } 289 290 long seconds = time / 1000L; 291 long milliseconds = time - seconds * 1000L; 292 seconds += OFFSET_1900_TO_1970; 293 294 // write seconds in big endian format 295 buffer[offset++] = (byte)(seconds >> 24); 296 buffer[offset++] = (byte)(seconds >> 16); 297 buffer[offset++] = (byte)(seconds >> 8); 298 buffer[offset++] = (byte)(seconds >> 0); 299 300 long fraction = milliseconds * 0x100000000L / 1000L; 301 // write fraction in big endian format 302 buffer[offset++] = (byte)(fraction >> 24); 303 buffer[offset++] = (byte)(fraction >> 16); 304 buffer[offset++] = (byte)(fraction >> 8); 305 // low order bits should be random data 306 buffer[offset++] = (byte)(Math.random() * 255.0); 307 } 308 } 309