How WebRTC Implements Video NACK: A Deep Dive into RTP Retransmission

This article explains the implementation of NACK on the WebRTC video sending side, based on the master branch code (commit f412945f05ce1ac372a7dad77d85498d23deaae).

Concept Overview

ACK is an acknowledgment that confirms receipt of data, while NACK is a negative acknowledgment that notifies the sender of missing packets. RFC4585 defines two types of retransmission for lost data, and the commonly used RTP packet loss retransmission offers a short recovery cycle with minimal bandwidth impact.

Key Implementation Steps

1. Send RTP packets and store them in the packet_history_ queue

Each time the pacer sends a packet, the media packet is stored in the packet_history_ queue.

ProcessThreadImpl::Process->PacedSender::Process->PacingController::ProcessPackets->PacketRouter::SendPacket->ModuleRtpRtcpImpl2::TrySendPacket->RtpSenderEgress::SendPacket

The RtpPacketHistory::PutRtpPacket method stores RTP packets indexed by sequence number.

The queue length is configured via SetStorePacketsStatus. Video streams are configured in CreateRtpStreamSenders->SetStorePacketsStatus, and audio streams in

RegisterSenderCongestionControlObjects->SetStorePacketsStatus

.

2. Handle received RTCP NACK packets

The call chain is:

RTCPReceiver::HandleNack -> push packet_information->nack_sequence_numbers into loss queue

Then ModuleRtpRtcpImpl::OnReceivedNack updates RTT and forwards the NACK sequence numbers to RTPSender::OnReceivedNack.

3. Retransmit RTP packets in response to NACK

RTPSender::OnReceivedNack

triggers retransmission. Three important points:

NackModule2::AddPacketsToNack decides whether a packet should be placed into the NACK queue.

Retransmission modes : either via a separate RTX channel or mixed with regular media; using a dedicated RTX channel is recommended to avoid interference with bitrate estimation and FEC configuration.

RTPSender::ReSendPacket enqueues the retransmitted packet with high priority using set_packet_type and the kRetransmission type.

Priority configuration is performed in GetPriorityForType, which assigns a high priority to retransmission packets, and the packet is then added to the pacer queue by PacingController::EnqueuePacket.