Consumer: add target layer retransmission buffer

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - libuv: Update to v1.51.0 ([PR #1543](https://github.com/versatica/mediasoup/pull/1543)).
 - libsrtp: Update to v3.0.0-beta version in our fork ([PR #1544](https://github.com/versatica/mediasoup/pull/1544)).
+- `XxxxConsumer`: Add target layer retransmission buffer ([PR #1548](https://github.com/versatica/mediasoup/pull/1548)).
 
 ### 3.16.0
 

worker/include/RTC/SimpleConsumer.hpp

@@ -5,6 +5,7 @@
 #include "RTC/Consumer.hpp"
 #include "RTC/SeqManager.hpp"
 #include "RTC/Shared.hpp"
+#include <map>
 
 namespace RTC
 {
@@ -72,6 +73,8 @@ namespace RTC
 		void UserOnResumed() override;
 		void CreateRtpStream();
 		void RequestKeyFrame();
+		void StorePacketInTargetLayerRetransmissionBuffer(
+		  RTC::RtpPacket* packet, std::shared_ptr<RTC::RtpPacket>& sharedPacket);
 		void EmitScore() const;
 
 		/* Pure virtual methods inherited from RtpStreamSend::Listener. */
@@ -90,6 +93,10 @@ namespace RTC
 		std::unique_ptr<RTC::SeqManager<uint16_t>> rtpSeqManager;
 		bool managingBitrate{ false };
 		std::unique_ptr<RTC::Codecs::EncodingContext> encodingContext;
+		// Buffer to store packets that arrive earlier than the first packet of the
+		// video key frame.
+		std::map<uint16_t, std::shared_ptr<RTC::RtpPacket>, RTC::SeqManager<uint16_t>::SeqLowerThan>
+		  targetLayerRetransmissionBuffer;
 	};
 } // namespace RTC
 

worker/src/RTC/SimpleConsumer.cpp

@@ -11,6 +11,10 @@
 
 namespace RTC
 {
+	/* Static. */
+
+	static constexpr size_t TargetLayerRetransmissionBufferSize{ 20u };
+
 	/* Instance methods. */
 
 	SimpleConsumer::SimpleConsumer(
@@ -74,6 +78,7 @@ namespace RTC
 		this->shared->channelMessageRegistrator->UnregisterHandler(this->id);
 
 		delete this->rtpStream;
+		this->targetLayerRetransmissionBuffer.clear();
 	}
 
 	flatbuffers::Offset<FBS::Consumer::DumpResponse> SimpleConsumer::FillBuffer(
@@ -362,31 +367,32 @@ namespace RTC
 			return;
 		}
 
-		// If we need to sync, support key frames and this is not a key frame, ignore
-		// the packet.
-		if (this->syncRequired && this->keyFrameSupported && !packet->IsKeyFrame())
+		// Packets with only padding are not forwarded.
+		if (packet->GetPayloadLength() == 0)
 		{
 #ifdef MS_RTC_LOGGER_RTP
-			packet->logger.Dropped(RtcLogger::RtpPacket::DropReason::NOT_A_KEYFRAME);
+			packet->logger.Dropped(RtcLogger::RtpPacket::DropReason::EMPTY_PAYLOAD);
 #endif
 
 			this->rtpSeqManager->Drop(packet->GetSequenceNumber());
 
 			return;
 		}
 
-		// Packets with only padding are not forwarded.
-		if (packet->GetPayloadLength() == 0)
+		// If we need to sync, support key frames and this is not a key frame, do not
+		// send the packet and store it for the scenario in which this packet is part
+		// of the key frame and arrived before the first packet of the key frame.
+		if (this->syncRequired && this->keyFrameSupported && !packet->IsKeyFrame())
 		{
-#ifdef MS_RTC_LOGGER_RTP
-			packet->logger.Dropped(RtcLogger::RtpPacket::DropReason::EMPTY_PAYLOAD);
-#endif
-
-			this->rtpSeqManager->Drop(packet->GetSequenceNumber());
+			StorePacketInTargetLayerRetransmissionBuffer(packet, sharedPacket);
 
 			return;
 		}
 
+		// Whether packets stored in the target layer retransmission buffer must be
+		// sent once this packet is sent.
+		bool sendPacketsInTargetLayerRetransmissionBuffer{ false };
+
 		// Whether this is the first packet after re-sync.
 		const bool isSyncPacket = this->syncRequired;
 
@@ -395,7 +401,13 @@ namespace RTC
 		{
 			if (packet->IsKeyFrame())
 			{
-				MS_DEBUG_TAG(rtp, "sync key frame received");
+				MS_DEBUG_TAG(
+				  rtp,
+				  "sync key frame received [seq:%" PRIu16 ", ts:%" PRIu32 "]",
+				  packet->GetSequenceNumber(),
+				  packet->GetTimestamp());
+
+				sendPacketsInTargetLayerRetransmissionBuffer = true;
 			}
 
 			this->rtpSeqManager->Sync(packet->GetSequenceNumber() - 1);
@@ -457,6 +469,28 @@ namespace RTC
 		// Restore packet fields.
 		packet->SetSsrc(origSsrc);
 		packet->SetSequenceNumber(origSeq);
+
+		// If sent packet was the first packet of a key frame, we may need to send
+		// buffered packets belonging to the same key frame that arrived earlier due
+		// to packet misorder.
+		if (sendPacketsInTargetLayerRetransmissionBuffer)
+		{
+			for (auto& pair : this->targetLayerRetransmissionBuffer)
+			{
+				auto& bufferedSharedPacket = pair.second;
+				auto* bufferedPacket       = bufferedSharedPacket.get();
+
+				MS_DEBUG_DEV(
+				  "sending packet buffered in the target layer retransmission buffer [seq:%" PRIu16
+				  ", ts:%" PRIu32 "]",
+				  bufferedPacket->GetSequenceNumber(),
+				  bufferedPacket->GetTimestamp());
+
+				SendRtpPacket(bufferedPacket, bufferedSharedPacket);
+			}
+
+			this->targetLayerRetransmissionBuffer.clear();
+		}
 	}
 
 	bool SimpleConsumer::GetRtcp(RTC::RTCP::CompoundPacket* packet, uint64_t nowMs)
@@ -607,13 +641,15 @@ namespace RTC
 		MS_TRACE();
 
 		this->rtpStream->Pause();
+		this->targetLayerRetransmissionBuffer.clear();
 	}
 
 	void SimpleConsumer::UserOnPaused()
 	{
 		MS_TRACE();
 
 		this->rtpStream->Pause();
+		this->targetLayerRetransmissionBuffer.clear();
 
 		if (this->externallyManagedBitrate && this->kind == RTC::Media::Kind::VIDEO)
 		{
@@ -729,6 +765,31 @@ namespace RTC
 		this->listener->OnConsumerKeyFrameRequested(this, mappedSsrc);
 	}
 
+	void SimpleConsumer::StorePacketInTargetLayerRetransmissionBuffer(
+	  RTC::RtpPacket* packet, std::shared_ptr<RTC::RtpPacket>& sharedPacket)
+	{
+		MS_TRACE();
+
+		MS_DEBUG_DEV(
+		  "storing packet in target layer retransmission buffer [seq:%" PRIu16 ", ts:%" PRIu32 "]",
+		  packet->GetSequenceNumber(),
+		  packet->GetTimestamp());
+
+		// Store original packet into the buffer. Only clone once and only if
+		// necessary.
+		if (!sharedPacket)
+		{
+			sharedPacket.reset(packet->Clone());
+		}
+
+		this->targetLayerRetransmissionBuffer[packet->GetSequenceNumber()] = sharedPacket;
+
+		if (this->targetLayerRetransmissionBuffer.size() > TargetLayerRetransmissionBufferSize)
+		{
+			this->targetLayerRetransmissionBuffer.erase(this->targetLayerRetransmissionBuffer.begin());
+		}
+	}
+
 	inline void SimpleConsumer::EmitScore() const
 	{
 		MS_TRACE();