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  /*
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   * Copyright (C) 2008 The Android Open Source Project
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   *
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   * Licensed under the Apache License, Version 2.0 (the "License");
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   * you may not use this file except in compliance with the License.
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   * You may obtain a copy of the License at
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   *
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   *      http://www.apache.org/licenses/LICENSE-2.0
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   *
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  * Unless required by applicable law or agreed to in writing, software
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  * distributed under the License is distributed on an "AS IS" BASIS,
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  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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  * See the License for the specific language governing permissions and
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  * limitations under the License.
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  */
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 package android.net;
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Simple SNTP client class for retrieving network time. Sample usage: 
SntpClient client = new SntpClient();
 if (client.requestTime("time.foo.com")) {
     long now = client.getNtpTime() + SystemClock.elapsedRealtime() - client.getNtpTimeReference();
 }
 
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 public class SntpClient
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 {
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     private static final String TAG = "SntpClient";
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     private static final int REFERENCE_TIME_OFFSET = 16;
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     private static final int ORIGINATE_TIME_OFFSET = 24;
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     private static final int RECEIVE_TIME_OFFSET = 32;
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     private static final int TRANSMIT_TIME_OFFSET = 40;
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     private static final int NTP_PACKET_SIZE = 48;
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     private static final int NTP_PORT = 123;
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     private static final int NTP_MODE_CLIENT = 3;
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     private static final int NTP_VERSION = 3;
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     // Number of seconds between Jan 1, 1900 and Jan 1, 1970
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     // 70 years plus 17 leap days
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     private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L;
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     // system time computed from NTP server response
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     private long mNtpTime;
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     // value of SystemClock.elapsedRealtime() corresponding to mNtpTime
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     private long mNtpTimeReference;
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     // round trip time in milliseconds
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     private long mRoundTripTime;

    
Sends an SNTP request to the given host and processes the response.

Parameters:
host host name of the server.
timeout network timeout in milliseconds.
Returns:
true if the transaction was successful.
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     public boolean requestTime(String host, int timeout) {
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         DatagramSocket socket = null;
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         try {
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             socket = new DatagramSocket();
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             socket.setSoTimeout(timeout);
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             InetAddress address = InetAddress.getByName(host);
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             byte[] buffer = new byte[];
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             DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, );
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             // set mode = 3 (client) and version = 3
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             // mode is in low 3 bits of first byte
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             // version is in bits 3-5 of first byte
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             buffer[0] =  | ( << 3);
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             // get current time and write it to the request packet
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             long requestTime = System.currentTimeMillis();
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             long requestTicks = SystemClock.elapsedRealtime();
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             writeTimeStamp(buffer, , requestTime);
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             socket.send(request);
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             // read the response
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             DatagramPacket response = new DatagramPacket(buffer, buffer.length);
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             socket.receive(response);
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             long responseTicks = SystemClock.elapsedRealtime();
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             long responseTime = requestTime + (responseTicks - requestTicks);
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            // extract the results
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            long originateTime = readTimeStamp(buffer, );
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            long receiveTime = readTimeStamp(buffer, );
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            long transmitTime = readTimeStamp(buffer, );
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            long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime);
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            // receiveTime = originateTime + transit + skew
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            // responseTime = transmitTime + transit - skew
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            // clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2
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            //             = ((originateTime + transit + skew - originateTime) +
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            //                (transmitTime - (transmitTime + transit - skew)))/2
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            //             = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2
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            //             = (transit + skew - transit + skew)/2
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            //             = (2 * skew)/2 = skew
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            long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2;
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            // if (false) Log.d(TAG, "round trip: " + roundTripTime + " ms");
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            // if (false) Log.d(TAG, "clock offset: " + clockOffset + " ms");
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            // save our results - use the times on this side of the network latency
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            // (response rather than request time)
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             = responseTime + clockOffset;
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             = responseTicks;
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             = roundTripTime;
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        } catch (Exception e) {
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            if (false) Log.d(, "request time failed: " + e);
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            return false;
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        } finally {
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            if (socket != null) {
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                socket.close();
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            }
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        }
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        return true;
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    }

    
Returns the time computed from the NTP transaction.

Returns:
time value computed from NTP server response.
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    public long getNtpTime() {
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        return ;
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    }

    
Returns the reference clock value (value of SystemClock.elapsedRealtime()) corresponding to the NTP time.

Returns:
reference clock corresponding to the NTP time.
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    public long getNtpTimeReference() {
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        return ;
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    }

    
Returns the round trip time of the NTP transaction

Returns:
round trip time in milliseconds.
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    public long getRoundTripTime() {
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        return ;
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    }

    
Reads an unsigned 32 bit big endian number from the given offset in the buffer. 
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    private long read32(byte[] buffer, int offset) {
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        byte b0 = buffer[offset];
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        byte b1 = buffer[offset+1];
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        byte b2 = buffer[offset+2];
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        byte b3 = buffer[offset+3];
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        // convert signed bytes to unsigned values
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        int i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0);
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        int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1);
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        int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2);
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        int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3);
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        return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3;
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    }

    
Reads the NTP time stamp at the given offset in the buffer and returns it as a system time (milliseconds since January 1, 1970). 
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    private long readTimeStamp(byte[] buffer, int offset) {
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        long seconds = read32(buffer, offset);
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        long fraction = read32(buffer, offset + 4);
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        return ((seconds - ) * 1000) + ((fraction * 1000L) / 0x100000000L);        
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    }

    
Writes system time (milliseconds since January 1, 1970) as an NTP time stamp at the given offset in the buffer. 
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    private void writeTimeStamp(byte[] buffer, int offset, long time) {
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        long seconds = time / 1000L;
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        long milliseconds = time - seconds * 1000L;
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        seconds += ;
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        // write seconds in big endian format
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        buffer[offset++] = (byte)(seconds >> 24);
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        buffer[offset++] = (byte)(seconds >> 16);
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        buffer[offset++] = (byte)(seconds >> 8);
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        buffer[offset++] = (byte)(seconds >> 0);
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        long fraction = milliseconds * 0x100000000L / 1000L;
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        // write fraction in big endian format
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        buffer[offset++] = (byte)(fraction >> 24);
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        buffer[offset++] = (byte)(fraction >> 16);
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        buffer[offset++] = (byte)(fraction >> 8);
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        // low order bits should be random data
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        buffer[offset++] = (byte)(Math.random() * 255.0);
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    }
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