Summary

Complete the PTO (Probe Timeout) handling in QUIC loss detection as required by RFC 9002 Section 6.2.

Current State

The connection.cpp:1105 has a TODO comment indicating PTO timeout loss detection is not fully implemented:

// TODO: Handle PTO timeout for loss detection

The loss_detector class has the infrastructure but the integration in connection is incomplete.

RFC 9002 Requirements

Per Section 6.2 (Probe Timeout):

• When PTO expires, send one or two probe packets
• Probe packets must be ack-eliciting
• Increase PTO count on each expiry
• Reset PTO count when receiving acknowledgments

Implementation Details

Current loss_detector API

class loss_detector {
public:
    [[nodiscard]] auto on_timeout() -> loss_detection_result;
    [[nodiscard]] auto pto_count() const noexcept -> uint32_t;
    auto reset_pto_count() noexcept -> void;
};

Required Changes in connection.cpp

void connection::on_timeout() {
    auto result = loss_detector_.on_timeout();
    
    if (result.event == loss_detection_event::pto_expired) {
        // 1. Generate probe packets
        generate_probe_packets();
        
        // 2. Handle lost packets
        for (const auto& lost : result.lost_packets) {
            retransmit_frames(lost.frames);
        }
        
        // 3. Update congestion controller
        for (const auto& lost : result.lost_packets) {
            congestion_controller_.on_packet_lost(lost);
        }
    }
}

Tasks

• Implement generate_probe_packets() method
• Connect on_timeout() with loss detection result handling
• Retransmit frames from lost packets
• Update congestion controller on loss
• Add unit tests for PTO timeout scenarios
• Add integration tests for packet loss recovery

Acceptance Criteria

• PTO timeout triggers probe packet generation
• Lost packets are properly retransmitted
• Congestion controller is notified of losses
• PTO count increments correctly
• Unit tests cover all PTO scenarios
• No regression in existing QUIC tests

Files to Modify

• src/protocols/quic/connection.cpp
• include/kcenon/network/protocols/quic/connection.h
• tests/test_quic_connection.cpp
• tests/test_quic_loss_detection.cpp

Related

• Parent Epic: [EPIC] network_system Feature Enhancement - Protocol Completion and Reliability #397
• RFC 9002 Section 6.2: Probe Timeout