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 {
InvalidServerReplyException(String message)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      */
requestTime(String host, int timeout, Network network)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 
requestTime(InetAddress address, int port, int timeout, Network network)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
requestTime(String host, int timeout)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      */
getNtpTime()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      */
getNtpTimeReference()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      */
getRoundTripTime()207     public long getRoundTripTime() {
208         return mRoundTripTime;
209     }
210 
checkValidServerReply( byte leap, byte mode, int stratum, long transmitTime)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      */
read32(byte[] buffer, int offset)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      */
readTimeStamp(byte[] buffer, int offset)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      */
writeTimeStamp(byte[] buffer, int offset, long time)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