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/*
* Copyright 2014-2025 Real Logic Limited.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package io.aeron.driver;
import io.aeron.protocol.DataHeaderFlyweight;
import org.agrona.concurrent.status.AtomicCounter;
import org.agrona.concurrent.NanoClock;
/**
* Tracking and handling of retransmit request, NAKs, for senders, and receivers.
*
* When configured for multicast, a max number of retransmits is permitted by
* {@link Configuration#MAX_RESEND_DEFAULT}. Additional received NAKs will be ignored if this maximum is reached.
* When configured for unicast, a single outstanding retransmit is permitted, and additional received NAKs
* will be ignored iff they overlap the current retransmit - otherwise the previous retransmit is assumed to have
* 'worked' and the new NAK will take its place.
*/
public final class RetransmitHandler
{
private final RetransmitAction[] retransmitActionPool;
private final NanoClock nanoClock;
private final FeedbackDelayGenerator delayGenerator;
private final FeedbackDelayGenerator lingerTimeoutGenerator;
private final AtomicCounter invalidPackets;
private final boolean hasGroupSemantics;
private final AtomicCounter retransmitOverflowCounter;
private int activeRetransmitCount = 0;
/**
* Create a handler for the dealing with the reception of frame request a frame to be retransmitted.
*
* @param nanoClock used to determine time.
* @param invalidPackets for recording invalid packets.
* @param delayGenerator to use for delay determination.
* @param lingerTimeoutGenerator to use for linger timeout.
* @param hasGroupSemantics indicates multicast/MDC semantics.
* @param maxRetransmits max retransmits for when group semantics is enabled
* @param retransmitOverflowCounter counter to track overflows.
*/
public RetransmitHandler(
final NanoClock nanoClock,
final AtomicCounter invalidPackets,
final FeedbackDelayGenerator delayGenerator,
final FeedbackDelayGenerator lingerTimeoutGenerator,
final boolean hasGroupSemantics,
final int maxRetransmits,
final AtomicCounter retransmitOverflowCounter)
{
this.nanoClock = nanoClock;
this.invalidPackets = invalidPackets;
this.delayGenerator = delayGenerator;
this.lingerTimeoutGenerator = lingerTimeoutGenerator;
this.hasGroupSemantics = hasGroupSemantics;
this.retransmitOverflowCounter = retransmitOverflowCounter;
final int actualMaxRetransmits = this.hasGroupSemantics ? maxRetransmits : 1;
retransmitActionPool = new RetransmitAction[actualMaxRetransmits];
for (int i = 0; i < actualMaxRetransmits; i++)
{
retransmitActionPool[i] = new RetransmitAction();
}
}
/**
* Called on reception of a NAK to start retransmit handling.
*
* @param termId from the NAK and the term id of the buffer to retransmit from.
* @param termOffset from the NAK and the offset of the data to retransmit.
* @param length of the missing data.
* @param termLength of the term buffer.
* @param mtuLength for the publication.
* @param flowControl for the publication (to clamp the retransmission length).
* @param retransmitSender to call if an immediate retransmit is required.
*/
public void onNak(
final int termId,
final int termOffset,
final int length,
final int termLength,
final int mtuLength,
final FlowControl flowControl,
final RetransmitSender retransmitSender)
{
if (!isInvalid(termOffset, termLength))
{
final int retransmitLength = flowControl.maxRetransmissionLength(termOffset, length, termLength, mtuLength);
final RetransmitAction action = scanForAvailableRetransmit(termId, termOffset, retransmitLength);
if (null != action)
{
action.termId = termId;
action.termOffset = termOffset;
action.length = retransmitLength;
final long delay = delayGenerator.generateDelayNs();
if (0 == delay)
{
retransmitSender.resend(termId, termOffset, action.length);
action.linger(lingerTimeoutGenerator.generateDelayNs(), nanoClock.nanoTime());
}
else
{
action.delay(delay, nanoClock.nanoTime());
}
}
}
}
/**
* Called to indicate a retransmission is received that may obviate the need to send one ourselves.
*
* NOTE: Currently only called from unit tests. Would be used for retransmitting from receivers for NAK suppression.
*
* @param termId of the data.
* @param termOffset of the data.
*/
public void onRetransmitReceived(final int termId, final int termOffset)
{
final RetransmitAction action = scanForExistingRetransmit(termId, termOffset);
if (null != action && RetransmitAction.State.DELAYED == action.state)
{
removeRetransmit(action);
}
}
/**
* Called to process any outstanding timeouts.
*
* @param nowNs time in nanoseconds.
* @param retransmitSender to call on retransmissions.
*/
public void processTimeouts(final long nowNs, final RetransmitSender retransmitSender)
{
if (activeRetransmitCount > 0)
{
for (final RetransmitAction action : retransmitActionPool)
{
if (RetransmitAction.State.DELAYED == action.state && (action.expiryNs - nowNs < 0))
{
retransmitSender.resend(action.termId, action.termOffset, action.length);
action.linger(lingerTimeoutGenerator.generateDelayNs(), nanoClock.nanoTime());
}
else if (RetransmitAction.State.LINGERING == action.state && (action.expiryNs - nowNs < 0))
{
removeRetransmit(action);
}
}
}
}
private boolean isInvalid(final int termOffset, final int termLength)
{
final boolean isInvalid = (termOffset > (termLength - DataHeaderFlyweight.HEADER_LENGTH)) || (termOffset < 0);
if (isInvalid)
{
invalidPackets.increment();
}
return isInvalid;
}
private RetransmitAction scanForAvailableRetransmit(final int termId, final int termOffset, final int length)
{
if (0 == activeRetransmitCount)
{
return addRetransmit(retransmitActionPool[0]);
}
RetransmitAction availableAction = null;
for (final RetransmitAction action : retransmitActionPool)
{
switch (action.state)
{
case INACTIVE:
if (null == availableAction)
{
availableAction = action;
}
break;
case DELAYED:
case LINGERING:
if (action.termId == termId &&
action.termOffset <= termOffset && termOffset < action.termOffset + action.length)
{
return null;
}
if (!hasGroupSemantics)
{
// this is unicast, and the NAK does NOT overlap the previous one, so just reuse it
availableAction = action;
}
break;
}
}
if (hasGroupSemantics)
{
if (null != availableAction)
{
return addRetransmit(availableAction);
}
retransmitOverflowCounter.increment();
}
return availableAction;
}
private RetransmitAction scanForExistingRetransmit(final int termId, final int termOffset)
{
if (0 == activeRetransmitCount)
{
return null;
}
for (final RetransmitAction action : retransmitActionPool)
{
switch (action.state)
{
case DELAYED:
case LINGERING:
if (action.termId == termId && action.termOffset == termOffset)
{
return action;
}
break;
default:
break;
}
}
return null;
}
private RetransmitAction addRetransmit(final RetransmitAction retransmitAction)
{
++activeRetransmitCount;
return retransmitAction;
}
private void removeRetransmit(final RetransmitAction action)
{
--activeRetransmitCount;
action.cancel();
}
static final class RetransmitAction
{
@SuppressWarnings("JavadocVariable")
enum State
{
DELAYED,
LINGERING,
INACTIVE
}
long expiryNs;
int termId;
int termOffset;
int length;
State state = State.INACTIVE;
void delay(final long delayNs, final long nowNs)
{
state = State.DELAYED;
expiryNs = nowNs + delayNs;
}
void linger(final long timeoutNs, final long nowNs)
{
state = State.LINGERING;
expiryNs = nowNs + timeoutNs;
}
void cancel()
{
state = State.INACTIVE;
}
}
}