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Retry alternative addresses on timeout in SNTP client
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Changed the default timeout for SNTP requests to 1 second.

Issue: #1540
PiperOrigin-RevId: 652897579
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@rohitjoins @copybara-github
rohitjoins authored and copybara-github committed Jul 16, 2024
1 parent 7f30409 commit ded66de
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Showing 2 changed files with 77 additions and 51 deletions.
4 changes: 2 additions & 2 deletions RELEASENOTES.md
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Expand Up @@ -24,8 +24,8 @@
* `MediaCodecVideoRenderer` avoids decoding samples that are neither
rendered nor used as reference by other samples.
* Add `BasePreloadManager.Listener` to propagate preload events to apps.
* Allow changing SNTP client timeout
([#1540](https://github.com/androidx/media/issues/1540)).
* Allow changing SNTP client timeout and retry alternative addresses on
timeout ([#1540](https://github.com/androidx/media/issues/1540)).
* Remove `MediaCodecAdapter.Configuration.flags` as the field was always
zero.
* Transformer:
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124 changes: 75 additions & 49 deletions libraries/exoplayer/src/main/java/androidx/media3/exoplayer/util/SntpClient.java
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Expand Up @@ -15,6 +15,8 @@
*/
package androidx.media3.exoplayer.util;

import static androidx.media3.common.util.Assertions.checkNotNull;

import android.os.SystemClock;
import androidx.annotation.GuardedBy;
import androidx.annotation.Nullable;
Expand All @@ -27,6 +29,7 @@
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketTimeoutException;
import java.util.Arrays;
import java.util.ConcurrentModificationException;

Expand All @@ -44,7 +47,7 @@ public final class SntpClient {
public static final String DEFAULT_NTP_HOST = "time.android.com";

/** The default maximum time, in milliseconds, to wait for the SNTP request to complete. */
public static final int DEFAULT_TIMEOUT_MS = 5_000;
public static final int DEFAULT_TIMEOUT_MS = 1_000;

/** Callback for calls to {@link #initialize(Loader, InitializationCallback)}. */
public interface InitializationCallback {
Expand All @@ -60,6 +63,7 @@ public interface InitializationCallback {
void onInitializationFailed(IOException error);
}

private static final int MAX_RETRY_COUNT = 10;
private static final int ORIGINATE_TIME_OFFSET = 24;
private static final int RECEIVE_TIME_OFFSET = 32;
private static final int TRANSMIT_TIME_OFFSET = 40;
Expand Down Expand Up @@ -191,56 +195,78 @@ public static void initialize(
}

private static long loadNtpTimeOffsetMs() throws IOException {
InetAddress address = InetAddress.getByName(getNtpHost());
try (DatagramSocket socket = new DatagramSocket()) {
socket.setSoTimeout(getTimeoutMs());
byte[] buffer = new byte[NTP_PACKET_SIZE];
DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT);

// Set mode = 3 (client) and version = 3. Mode is in low 3 bits of the first byte and Version
// is in bits 3-5 of the first byte.
buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3);

// Get current time and write it to the request packet.
long requestTime = System.currentTimeMillis();
long requestTicks = SystemClock.elapsedRealtime();
writeTimestamp(buffer, TRANSMIT_TIME_OFFSET, requestTime);

socket.send(request);

// Read the response.
DatagramPacket response = new DatagramPacket(buffer, buffer.length);
socket.receive(response);
final long responseTicks = SystemClock.elapsedRealtime();
final long responseTime = requestTime + (responseTicks - requestTicks);

// Extract the results.
final byte leap = (byte) ((buffer[0] >> 6) & 0x3);
final byte mode = (byte) (buffer[0] & 0x7);
final int stratum = (int) (buffer[1] & 0xff);
final long originateTime = readTimestamp(buffer, ORIGINATE_TIME_OFFSET);
final long receiveTime = readTimestamp(buffer, RECEIVE_TIME_OFFSET);
final long transmitTime = readTimestamp(buffer, TRANSMIT_TIME_OFFSET);

// Check server reply validity according to RFC.
checkValidServerReply(leap, mode, stratum, transmitTime);

// receiveTime = originateTime + transit + skew
// responseTime = transmitTime + transit - skew
// clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2
// = ((originateTime + transit + skew - originateTime) +
// (transmitTime - (transmitTime + transit - skew)))/2
// = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2
// = (transit + skew - transit + skew)/2
// = (2 * skew)/2 = skew
long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime)) / 2;

// Save our results using the times on this side of the network latency (i.e. response rather
// than request time)
long ntpTime = responseTime + clockOffset;
long ntpTimeReference = responseTicks;

return ntpTime - ntpTimeReference;

int retryCount = 0;
SocketTimeoutException timeoutException = null;
InetAddress[] addresses = InetAddress.getAllByName(getNtpHost());
for (InetAddress address : addresses) {
byte[] buffer = new byte[NTP_PACKET_SIZE];
DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT);

// Set mode = 3 (client) and version = 3. Mode is in low 3 bits of the first byte and
// Version is in bits 3-5 of the first byte.
buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3);

// Get current time and write it to the request packet.
long requestTime = System.currentTimeMillis();
long requestTicks = SystemClock.elapsedRealtime();
writeTimestamp(buffer, TRANSMIT_TIME_OFFSET, requestTime);

socket.send(request);

// Read the response.
DatagramPacket response = new DatagramPacket(buffer, buffer.length);
try {
socket.receive(response);
} catch (SocketTimeoutException e) {
// Store the timeout exception and try the next address if we have not reached retry limit
if (timeoutException == null) {
timeoutException = e;
} else {
timeoutException.addSuppressed(e);
}
if (retryCount++ < MAX_RETRY_COUNT) {
continue;
} else {
break;
}
}

final long responseTicks = SystemClock.elapsedRealtime();
final long responseTime = requestTime + (responseTicks - requestTicks);

// Extract the results.
final byte leap = (byte) ((buffer[0] >> 6) & 0x3);
final byte mode = (byte) (buffer[0] & 0x7);
final int stratum = (int) (buffer[1] & 0xff);
final long originateTime = readTimestamp(buffer, ORIGINATE_TIME_OFFSET);
final long receiveTime = readTimestamp(buffer, RECEIVE_TIME_OFFSET);
final long transmitTime = readTimestamp(buffer, TRANSMIT_TIME_OFFSET);

// Check server reply validity according to RFC.
checkValidServerReply(leap, mode, stratum, transmitTime);

// receiveTime = originateTime + transit + skew
// responseTime = transmitTime + transit - skew
// clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2
// = ((originateTime + transit + skew - originateTime) +
// (transmitTime - (transmitTime + transit - skew)))/2
// = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2
// = (transit + skew - transit + skew)/2
// = (2 * skew)/2 = skew
long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime)) / 2;

// Save our results using the times on this side of the network latency (i.e. response
// rather than request time)
long ntpTime = responseTime + clockOffset;
long ntpTimeReference = responseTicks;

return ntpTime - ntpTimeReference;
}
// If no response is received from any of the addresses, throw an exception.
throw checkNotNull(timeoutException);
}
}

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