Add configuration for HTTP/2 PING scheduler interval and retry threshold

- Added a method to configure the execution interval of the scheduler
  that sends HTTP/2 PING frames and periodically checks for ACK responses
- Introduced a retry threshold setting to limit the number of PING transmission attempts
  before considering the connection as unresponsive
- Default values:
  - Scheduler interval must be explicitly set
  - Retry threshold defaults to 0 (no retries, only one PING attempt)

Signed-off-by: raccoonback <kosb15@naver.com>

docs/modules/ROOT/pages/http-client.adoc

@@ -318,7 +318,10 @@ include::{examples-dir}/http2/H2Application.java[lines=18..42]
 ----
 <1> Configures the client to support only `HTTP/2`
 <2> Configures `SSL`
-<3> You can configure the interval for checking `Ping` frames
+<3> Sets the interval for sending `HTTP/2` `PING` frames and receiving `ACK` responses
+<4> Sets the execution interval for the scheduler that sends `HTTP/2` `PING frames and periodically checks for `ACK` responses
+<5> Sets the threshold for retrying `HTTP/2` `PING` frame transmissions.
+
 
 The following listing presents a simple `H2C` example:
 

reactor-netty-core/src/main/java/reactor/netty/NettyPipeline.java

@@ -111,7 +111,7 @@ public interface NettyPipeline {
 	String OnChannelReadIdle     = LEFT + "onChannelReadIdle";
 	String OnChannelWriteIdle    = LEFT + "onChannelWriteIdle";
 	String ProxyHandler          = LEFT + "proxyHandler";
-	String H2LivenessHandler          = LEFT + "h2LivenessHandler";
+	String H2LivenessHandler     = LEFT + "h2LivenessHandler";
 	/**
 	 * Use to register a special handler which ensures that any {@link io.netty.channel.VoidChannelPromise}
 	 * will be converted to "unvoided" promises.

reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/http2/H2Application.java

@@ -30,7 +30,11 @@ public static void main(String[] args) {
 				HttpClient.create()
 				          .protocol(HttpProtocol.H2) //<1>
 				          .secure()                 //<2>
-						  .http2Settings(builder -> builder.pingInterval(Duration.ofMillis(100))); // <3>
+						  .http2Settings(
+								  builder -> builder.pingAckTimeout(Duration.ofMillis(600))  // <3>
+										  .pingScheduleInterval(Duration.ofMillis(300))  // <4>
+										  .pingAckDropThreshold(2)  // <5>
+						  );
 
 		Tuple2<String, HttpHeaders> response =
 				client.get()

reactor-netty-http/src/main/java/reactor/netty/http/Http2SettingsSpec.java

@@ -96,19 +96,75 @@ public interface Builder {
 		//Builder pushEnabled(boolean pushEnabled);
 
 		/**
-		 * Sets the interval for checking ping frames.
-		 * If a ping ACK frame is not received within the configured interval, the connection will be closed.
+		 * Sets the interval for sending HTTP/2 PING frames and receiving ACK responses.
 		 *
-		 * <p>Be cautious when setting a very short interval, as it may cause the connection to be closed,
-		 * even if the keep-alive setting is enabled.</p>
+		 * <p>
+		 * This method configures the time interval at which PING frames are sent to the peer.
+		 * The interval should be chosen carefully to balance between detecting connection issues
+		 * and minimizing unnecessary network traffic.
+		 * </p>
 		 *
-		 * <p>If no interval is specified, no ping frame checking will be performed by default.</p>
+		 * <p>
+		 * If the interval is set too short, it may cause excessive network overhead.
+		 * If set too long, connection failures may not be detected in a timely manner.
+		 * </p>
 		 *
-		 * @param pingInterval the duration between sending ping frames. If not specified, ping frame checking is disabled.
-		 * @return {@code this}
-		 * @since 1.2.3
+		 * @param pingAckTimeout the interval in between consecutive PING frames
+		 *                       and ACK responses. Must be a positive value.
+		 */
+		default Builder pingAckTimeout(Duration pingAckTimeout) {
+			return this;
+		}
+
+		/**
+		 * Sets the execution interval for the scheduler that sends HTTP/2 PING frames
+		 * and periodically checks for ACK responses.
+		 *
+		 * <p>
+		 * This method configures the time interval at which the scheduler runs
+		 * to send PING frames and verify if ACK responses are received within
+		 * the expected timeframe.
+		 * Proper tuning of this interval helps in detecting connection issues
+		 * while avoiding unnecessary network overhead.
+		 * </p>
+		 *
+		 * <p>
+		 * If the interval is too short, it may increase network and CPU usage.
+		 * Conversely, setting it too long may delay the detection of connection failures.
+		 * </p>
+		 *
+		 * @param pingScheduleInterval the interval in at which the scheduler executes.
+		 *                       Must be a positive value.
 		 */
-		default Builder pingInterval(Duration pingInterval) {
+		default Builder pingScheduleInterval(Duration pingScheduleInterval) {
+			return this;
+		}
+
+		/**
+		 * Sets the threshold for retrying HTTP/2 PING frame transmissions.
+		 *
+		 * <p>
+		 * This method defines the maximum number of attempts to send a PING frame
+		 * before considering the connection as unresponsive.
+		 * If the threshold is exceeded without receiving an ACK response,
+		 * the connection may be closed or marked as unhealthy.
+		 * </p>
+		 *
+		 * <p>
+		 * A lower threshold can detect connection failures more quickly but may lead
+		 * to premature disconnections. Conversely, a higher threshold allows more retries
+		 * but may delay failure detection.
+		 * </p>
+		 *
+		 * <p>
+		 * If this value is not specified, it defaults to 0, meaning only one attempt to send a PING frame is made without retries.
+		 * </p>
+		 *
+		 * @param pingAckDropThreshold the maximum number of PING transmission attempts.
+		 *                 Must be a positive integer.
+		 *                 The default value is 0, meaning no retries will occur and only one PING frame will be sent.
+		 */
+		default Builder pingAckDropThreshold(Integer pingAckDropThreshold) {
 			return this;
 		}
 	}
@@ -195,13 +251,33 @@ public Boolean pushEnabled() {
 	}
 
 	/**
-	 * Returns the configured {@code pingInterval} value or null.
+	 * Returns the configured {@code pingAckTimeout} value or null.
+	 *
+	 * @return the configured {@code pingAckTimeout} value or null
+	 */
+	@Nullable
+	public Duration pingAckTimeout() {
+		return pingAckTimeout;
+	}
+
+	/**
+	 * Returns the configured {@code pingScheduleInterval} value or null.
 	 *
-	 * @return the configured {@code pingInterval} value or null
+	 * @return the configured {@code pingScheduleInterval} value or null
 	 */
 	@Nullable
-	public Duration pingInterval() {
-		return pingInterval;
+	public Duration pingScheduleInterval() {
+		return pingScheduleInterval;
+	}
+
+	/**
+	 * Returns the configured {@code pingAckDropThreshold} value or null.
+	 *
+	 * @return the configured {@code pingAckDropThreshold} value or null
+	 */
+	@Nullable
+	public Integer pingAckDropThreshold() {
+		return pingAckDropThreshold;
 	}
 
 	@Override
@@ -220,7 +296,9 @@ public boolean equals(Object o) {
 				maxHeaderListSize.equals(that.maxHeaderListSize) &&
 				Objects.equals(maxStreams, that.maxStreams) &&
 				Objects.equals(pushEnabled, that.pushEnabled) &&
-				Objects.equals(pingInterval, that.pingInterval);
+				Objects.equals(pingAckTimeout, that.pingAckTimeout) &&
+				Objects.equals(pingScheduleInterval, that.pingScheduleInterval) &&
+				Objects.equals(pingAckDropThreshold, that.pingAckDropThreshold);
 	}
 
 	@Override
@@ -233,7 +311,9 @@ public int hashCode() {
 		result = 31 * result + Long.hashCode(maxHeaderListSize);
 		result = 31 * result + Long.hashCode(maxStreams);
 		result = 31 * result + Boolean.hashCode(pushEnabled);
-		result = 31 * result + Objects.hashCode(pingInterval);
+		result = 31 * result + Objects.hashCode(pingAckTimeout);
+		result = 31 * result + Objects.hashCode(pingScheduleInterval);
+		result = 31 * result + Objects.hashCode(pingAckDropThreshold);
 		return result;
 	}
 
@@ -244,7 +324,9 @@ public int hashCode() {
 	final Long maxHeaderListSize;
 	final Long maxStreams;
 	final Boolean pushEnabled;
-	final Duration pingInterval;
+	final Duration pingAckTimeout;
+	final Duration pingScheduleInterval;
+	final Integer pingAckDropThreshold;
 
 	Http2SettingsSpec(Build build) {
 		Http2Settings settings = build.http2Settings;
@@ -261,12 +343,16 @@ public int hashCode() {
 		maxHeaderListSize = settings.maxHeaderListSize();
 		maxStreams = build.maxStreams;
 		pushEnabled = settings.pushEnabled();
-		pingInterval = build.pingInterval;
+		pingAckTimeout = build.pingAckTimeout;
+		pingScheduleInterval = build.pingScheduleInterval;
+		pingAckDropThreshold = build.pingAckDropThreshold;
 	}
 
 	static final class Build implements Builder {
 		Long maxStreams;
-		Duration pingInterval;
+		Duration pingAckTimeout;
+		Duration pingScheduleInterval;
+		Integer pingAckDropThreshold;
 		final Http2Settings http2Settings = Http2Settings.defaultSettings();
 
 		@Override
@@ -314,8 +400,20 @@ public Builder maxStreams(long maxStreams) {
 		}
 
 		@Override
-		public Builder pingInterval(Duration pingInterval) {
-			this.pingInterval = pingInterval;
+		public Builder pingAckTimeout(Duration pingAckTimeout) {
+			this.pingAckTimeout = pingAckTimeout;
+			return this;
+		}
+
+		@Override
+		public Builder pingScheduleInterval(Duration pingScheduleInterval) {
+			this.pingScheduleInterval = pingScheduleInterval;
+			return this;
+		}
+
+		@Override
+		public Builder pingAckDropThreshold(Integer pingAckDropThreshold) {
+			this.pingAckDropThreshold = pingAckDropThreshold;
 			return this;
 		}
 

reactor-netty-http/src/main/java/reactor/netty/http/client/Http2ConnectionLivenessHandler.java

@@ -28,6 +28,7 @@
 import reactor.util.annotation.Nullable;
 
 import java.time.Duration;
+import java.util.Objects;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.ThreadLocalRandom;
 
@@ -55,33 +56,57 @@ final class Http2ConnectionLivenessHandler extends ChannelDuplexHandler {
 	private static final Logger log = Loggers.getLogger(Http2ConnectionLivenessHandler.class);
 
 	private ScheduledFuture<?> pingScheduler;
+
 	private final ChannelFutureListener pingWriteListener = new PingWriteListener();
 	private final Http2ConnectionEncoder encoder;
-	private final long pingIntervalNanos;
+	private final long pingAckTimeoutNanos;
+	private final long pingScheduleIntervalNanos;
+	private final int pingAckDropThreshold;
+
+	private int pingAckDropCount;
 	private long lastSentPingData;
 	private long lastReceivedPingTime;
+	private long lastSendingPingTime;
 	private long lastIoTime;
 	private boolean isPingAckPending;
 
-	public Http2ConnectionLivenessHandler(Http2ConnectionEncoder encoder, @Nullable Duration pingInterval) {
+	public Http2ConnectionLivenessHandler(Http2ConnectionEncoder encoder, @Nullable Duration pingAckTimeout,
+	                                      @Nullable Duration pintScheduleInterval, @Nullable Integer pingAckDropThreshold) {
+		Objects.requireNonNull(encoder, "encoder");
+
 		this.encoder = encoder;
 
-		if (pingInterval != null) {
-			this.pingIntervalNanos = pingInterval.toNanos();
+		if (pingAckTimeout != null) {
+			this.pingAckTimeoutNanos = pingAckTimeout.toNanos();
+		}
+		else {
+			this.pingAckTimeoutNanos = 0L;
+		}
+
+		if (pintScheduleInterval != null) {
+			this.pingScheduleIntervalNanos = pintScheduleInterval.toNanos();
 		}
 		else {
-			this.pingIntervalNanos = 0L;
+			this.pingScheduleIntervalNanos = 0L;
+		}
+
+		if (pingAckDropThreshold != null) {
+			this.pingAckDropThreshold = pingAckDropThreshold;
+		}
+		else {
+			this.pingAckDropThreshold = 0;
 		}
 	}
 
 	@Override
 	public void channelActive(ChannelHandlerContext ctx) throws Exception {
 		if (isPingIntervalConfigured()) {
 			isPingAckPending = false;
+			pingAckDropCount = 0;
 			pingScheduler = ctx.executor()
 					.schedule(
 							new PingChecker(ctx),
-							pingIntervalNanos,
+							pingAckTimeoutNanos,
 							NANOSECONDS
 					);
 		}
@@ -118,14 +143,9 @@ public void channelInactive(ChannelHandlerContext ctx) throws Exception {
 		ctx.fireChannelInactive();
 	}
 
-	@Override
-	public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
-		cancel();
-		ctx.fireExceptionCaught(cause);
-	}
-
 	private boolean isPingIntervalConfigured() {
-		return pingIntervalNanos > 0;
+		return pingAckTimeoutNanos > 0
+				&& pingScheduleIntervalNanos > 0;
 	}
 
 	private void cancel() {
@@ -155,6 +175,7 @@ public void run() {
 				}
 
 				isPingAckPending = false;
+				pingAckDropCount = 0;
 				pingScheduler = invokeNextSchedule();
 				return;
 			}
@@ -170,15 +191,28 @@ public void run() {
 			}
 
 			if (isOutOfTimeRange()) {
+				countPingDrop();
+
+				if (isExceedAckDropThreshold()) {
+					if (log.isInfoEnabled()) {
+						log.info("Closing {} channel due to delayed ping frame response (timeout: {} ns). lastReceivedPingTime: {}, current: {}", channel, pingAckTimeoutNanos, lastReceivedPingTime, System.nanoTime());
+					}
+
+					close(channel);
+					return;
+				}
+
 				if (log.isInfoEnabled()) {
-					log.info("Closing {} channel due to delayed ping frame response (timeout: {} ns).", channel, pingIntervalNanos);
+					log.info("Drop ping ack frame in {} channel. (ping: {})", channel, lastSentPingData);
 				}
 
-				close(channel);
+				writePing(ctx);
+				pingScheduler = invokeNextSchedule();
 				return;
 			}
 
 			isPingAckPending = false;
+			pingAckDropCount = 0;
 			pingScheduler = invokeNextSchedule();
 		}
 
@@ -192,18 +226,26 @@ private void writePing(ChannelHandlerContext ctx) {
 		}
 
 		private boolean isIoInProgress() {
-			return pingIntervalNanos > (System.nanoTime() - lastIoTime);
+			return pingAckTimeoutNanos >= (System.nanoTime() - lastIoTime);
 		}
 
 		private boolean isOutOfTimeRange() {
-			return pingIntervalNanos < (System.nanoTime() - lastReceivedPingTime);
+			return pingAckTimeoutNanos < Math.abs(lastReceivedPingTime - lastSendingPingTime);
+		}
+
+		private void countPingDrop() {
+			pingAckDropCount++;
+		}
+
+		private boolean isExceedAckDropThreshold() {
+			return pingAckDropCount > pingAckDropThreshold;
 		}
 
 		private ScheduledFuture<?> invokeNextSchedule() {
 			return ctx.executor()
 					.schedule(
 							new PingChecker(ctx),
-							pingIntervalNanos,
+							pingScheduleIntervalNanos,
 							NANOSECONDS
 					);
 		}
@@ -233,6 +275,7 @@ public void operationComplete(ChannelFuture future) throws Exception {
 				}
 
 				isPingAckPending = true;
+				lastSendingPingTime = System.nanoTime();
 			}
 			else if (log.isDebugEnabled()) {
 				log.debug("Failed to wrote PING frame to {} channel.", future.channel());