Add NTP client to get time offset for live streams without UTCTiming.
Dash live streams require that the client has an accurate wall clock time and in absence of a UTCTiming element, this is assumed to be the NTP time. This change adds NTP time offset resolution for DASH live streams without such timing elements. PiperOrigin-RevId: 289098796
This commit is contained in:
tonihei
Oliver Woodman
parent
3e08e42168
commit
01e661f21a
@ -0,0 +1,310 @@
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/*
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* Copyright (C) 2019 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 com.google.android.exoplayer2.util;
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import android.os.SystemClock;
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import androidx.annotation.GuardedBy;
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import androidx.annotation.Nullable;
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import com.google.android.exoplayer2.C;
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import com.google.android.exoplayer2.upstream.Loader;
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import com.google.android.exoplayer2.upstream.Loader.LoadErrorAction;
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import com.google.android.exoplayer2.upstream.Loader.Loadable;
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import java.io.IOException;
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import java.net.DatagramPacket;
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import java.net.DatagramSocket;
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import java.net.InetAddress;
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import java.util.Arrays;
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/**
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* Static utility to retrieve the device time offset using SNTP.
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*
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* <p>Based on the <a
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* href="https://cs.android.com/android/_/android/platform/frameworks/base/+/ea1b85a5eb6e9dcddf7abc6c74479bfadf9017c7:core/java/android/net/SntpClient.java">Android
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* framework SntpClient</a>.
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*/
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public final class SntpClient {
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/** Callback for calls to {@link #initialize(Loader, InitializationCallback)}. */
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public interface InitializationCallback {
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/** Called when the device time offset has been initialized. */
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void onInitialized();
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/**
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* Called when the device time offset failed to initialize.
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*
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* @param error The error that caused the initialization failure.
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*/
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void onInitializationFailed(IOException error);
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}
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private static final String NTP_HOST = "pool.ntp.org";
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private static final int TIMEOUT_MS = 10_000;
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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_MODE_SERVER = 4;
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private static final int NTP_MODE_BROADCAST = 5;
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private static final int NTP_VERSION = 3;
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private static final int NTP_LEAP_NOSYNC = 3;
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private static final int NTP_STRATUM_DEATH = 0;
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private static final int NTP_STRATUM_MAX = 15;
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private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L;
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private static final Object loaderLock = new Object();
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private static final Object valueLock = new Object();
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@GuardedBy("valueLock")
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private static boolean isInitialized;
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@GuardedBy("valueLock")
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private static long elapsedRealtimeOffsetMs;
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private SntpClient() {}
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/**
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* Returns whether the device time offset has already been loaded.
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*
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* <p>If {@code false}, use {@link #initialize(Loader, InitializationCallback)} to start the
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* initialization.
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*/
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public static boolean isInitialized() {
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synchronized (valueLock) {
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return isInitialized;
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}
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}
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/**
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* Returns the offset between {@link SystemClock#elapsedRealtime()} and the NTP server time in
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* milliseconds, or {@link C#TIME_UNSET} if {@link #isInitialized()} returns false.
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*
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* <p>The offset is calculated as {@code ntpServerTime - deviceElapsedRealTime}.
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*/
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public static long getElapsedRealtimeOffsetMs() {
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synchronized (valueLock) {
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return isInitialized ? elapsedRealtimeOffsetMs : C.TIME_UNSET;
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}
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}
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/**
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* Starts loading the device time offset.
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*
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* @param loader A {@link Loader} to use for loading the time offset, or null to create a new one.
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* @param callback An optional {@link InitializationCallback} to be notified when the time offset
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* has been initialized or initialization failed.
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*/
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public static void initialize(
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@Nullable Loader loader, @Nullable InitializationCallback callback) {
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if (isInitialized()) {
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if (callback != null) {
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callback.onInitialized();
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}
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return;
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}
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if (loader == null) {
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loader = new Loader("SntpClient");
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}
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loader.startLoading(
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new NtpTimeLoadable(), new NtpTimeCallback(callback), /* defaultMinRetryCount= */ 1);
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}
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private static long loadNtpTimeOffsetMs() throws IOException {
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InetAddress address = InetAddress.getByName(NTP_HOST);
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try (DatagramSocket socket = new DatagramSocket()) {
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socket.setSoTimeout(TIMEOUT_MS);
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byte[] buffer = new byte[NTP_PACKET_SIZE];
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DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT);
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// Set mode = 3 (client) and version = 3. Mode is in low 3 bits of the first byte and Version
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// is in bits 3-5 of the first byte.
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buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 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, TRANSMIT_TIME_OFFSET, 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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final long responseTicks = SystemClock.elapsedRealtime();
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final long responseTime = requestTime + (responseTicks - requestTicks);
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// Extract the results.
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final byte leap = (byte) ((buffer[0] >> 6) & 0x3);
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final byte mode = (byte) (buffer[0] & 0x7);
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final int stratum = (int) (buffer[1] & 0xff);
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final long originateTime = readTimestamp(buffer, ORIGINATE_TIME_OFFSET);
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final long receiveTime = readTimestamp(buffer, RECEIVE_TIME_OFFSET);
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final long transmitTime = readTimestamp(buffer, TRANSMIT_TIME_OFFSET);
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// Do sanity check according to RFC.
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checkValidServerReply(leap, mode, stratum, transmitTime);
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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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// Save our results using the times on this side of the network latency (i.e. response rather
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// than request time)
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long ntpTime = responseTime + clockOffset;
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long ntpTimeReference = responseTicks;
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return ntpTime - ntpTimeReference;
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}
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}
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private static 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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// Special case: zero means zero.
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if (seconds == 0 && fraction == 0) {
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return 0;
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}
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return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L);
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}
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private static void writeTimestamp(byte[] buffer, int offset, long time) {
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// Special case: zero means zero.
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if (time == 0) {
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Arrays.fill(buffer, offset, offset + 8, (byte) 0x00);
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return;
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}
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long seconds = time / 1000L;
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long milliseconds = time - seconds * 1000L;
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seconds += OFFSET_1900_TO_1970;
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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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private static 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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}
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private static void checkValidServerReply(byte leap, byte mode, int stratum, long transmitTime)
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throws IOException {
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if (leap == NTP_LEAP_NOSYNC) {
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throw new IOException("SNTP: Unsynchronized server");
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}
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if ((mode != NTP_MODE_SERVER) && (mode != NTP_MODE_BROADCAST)) {
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throw new IOException("SNTP: Untrusted mode: " + mode);
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}
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if ((stratum == NTP_STRATUM_DEATH) || (stratum > NTP_STRATUM_MAX)) {
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throw new IOException("SNTP: Untrusted stratum: " + stratum);
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}
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if (transmitTime == 0) {
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throw new IOException("SNTP: Zero transmitTime");
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}
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}
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private static final class NtpTimeLoadable implements Loadable {
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@Override
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public void cancelLoad() {}
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@Override
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public void load() throws IOException {
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// Synchronized to prevent redundant parallel requests.
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synchronized (loaderLock) {
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synchronized (valueLock) {
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if (isInitialized) {
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return;
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}
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}
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long offsetMs = loadNtpTimeOffsetMs();
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synchronized (valueLock) {
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elapsedRealtimeOffsetMs = offsetMs;
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isInitialized = true;
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}
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}
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}
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}
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private static final class NtpTimeCallback implements Loader.Callback<Loadable> {
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@Nullable private final InitializationCallback callback;
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public NtpTimeCallback(@Nullable InitializationCallback callback) {
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this.callback = callback;
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}
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@Override
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public void onLoadCompleted(Loadable loadable, long elapsedRealtimeMs, long loadDurationMs) {
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Assertions.checkState(SntpClient.isInitialized());
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if (callback != null) {
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callback.onInitialized();
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}
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}
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@Override
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public void onLoadCanceled(
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Loadable loadable, long elapsedRealtimeMs, long loadDurationMs, boolean released) {
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// Ignore.
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}
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@Override
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public LoadErrorAction onLoadError(
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Loadable loadable,
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long elapsedRealtimeMs,
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long loadDurationMs,
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IOException error,
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int errorCount) {
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if (callback != null) {
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callback.onInitializationFailed(error);
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}
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return Loader.DONT_RETRY;
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}
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}
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}
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@ -54,6 +54,7 @@ import com.google.android.exoplayer2.upstream.ParsingLoadable;
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import com.google.android.exoplayer2.upstream.TransferListener;
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import com.google.android.exoplayer2.util.Assertions;
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import com.google.android.exoplayer2.util.Log;
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import com.google.android.exoplayer2.util.SntpClient;
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import com.google.android.exoplayer2.util.Util;
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import java.io.BufferedReader;
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import java.io.IOException;
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@ -821,8 +822,12 @@ public final class DashMediaSource extends BaseMediaSource {
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}
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if (oldPeriodCount == 0) {
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if (manifest.dynamic && manifest.utcTiming != null) {
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if (manifest.dynamic) {
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if (manifest.utcTiming != null) {
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resolveUtcTimingElement(manifest.utcTiming);
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} else {
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loadNtpTimeOffset();
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}
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} else {
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processManifest(true);
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}
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@ -915,6 +920,9 @@ public final class DashMediaSource extends BaseMediaSource {
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} else if (Util.areEqual(scheme, "urn:mpeg:dash:utc:http-xsdate:2014")
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|| Util.areEqual(scheme, "urn:mpeg:dash:utc:http-xsdate:2012")) {
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resolveUtcTimingElementHttp(timingElement, new XsDateTimeParser());
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} else if (Util.areEqual(scheme, "urn:mpeg:dash:utc:ntp:2014")
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|| Util.areEqual(scheme, "urn:mpeg:dash:utc:ntp:2012")) {
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loadNtpTimeOffset();
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} else {
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// Unsupported scheme.
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onUtcTimestampResolutionError(new IOException("Unsupported UTC timing scheme"));
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@ -936,13 +944,29 @@ public final class DashMediaSource extends BaseMediaSource {
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C.DATA_TYPE_TIME_SYNCHRONIZATION, parser), new UtcTimestampCallback(), 1);
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}
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private void loadNtpTimeOffset() {
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SntpClient.initialize(
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loader,
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new SntpClient.InitializationCallback() {
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@Override
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public void onInitialized() {
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onUtcTimestampResolved(SntpClient.getElapsedRealtimeOffsetMs());
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}
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@Override
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public void onInitializationFailed(IOException error) {
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onUtcTimestampResolutionError(error);
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}
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});
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}
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private void onUtcTimestampResolved(long elapsedRealtimeOffsetMs) {
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this.elapsedRealtimeOffsetMs = elapsedRealtimeOffsetMs;
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processManifest(true);
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}
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private void onUtcTimestampResolutionError(IOException error) {
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Log.e(TAG, "Failed to resolve UtcTiming element.", error);
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Log.e(TAG, "Failed to resolve time offset.", error);
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// Be optimistic and continue in the hope that the device clock is correct.
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processManifest(true);
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}
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