766c9df442
- DredTuner: maps live network metrics (loss/RTT/jitter) to continuous DRED duration every ~500ms instead of discrete tier-locked values. Includes jitter-spike detection for pre-emptive Starlink-style boost. - Opus6k DRED extended from 500ms to 1040ms (max libopus 1.5 supports) - PMTUD: quinn MtuDiscoveryConfig with upper_bound=1452, 300s interval - TrunkedForwarder respects discovered MTU (was hard-coded 1200) - QuinnPathSnapshot exposes quinn internal stats + discovered MTU - AudioEncoder trait: set_expected_loss() + set_dred_duration() methods - PathMonitor: sliding-window jitter variance for spike detection - Integrated into both Android and desktop send tasks in engine.rs - 14 new tests (10 tuner unit + 4 encoder integration) - Updated ARCHITECTURE.md, PROGRESS.md, PRD-dred-integration, PRD-mtu Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
85 lines
3.3 KiB
Markdown
85 lines
3.3 KiB
Markdown
# PRD: QUIC Path MTU Discovery
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## Problem
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WarzonePhone uses conservative 1200-byte QUIC datagrams. Some network paths support larger MTUs (1400+), wasting bandwidth. Some broken paths (VPNs, tunnels, double-NAT, cellular) have MTU < 1200, causing silent packet drops — this may explain why Opus 64k fails on some paths while 24k works (larger encoded frames + FEC repair packets).
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## Solution
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Enable Quinn's built-in Path MTU Discovery (PMTUD) and handle edge cases:
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1. PMTUD probes larger packet sizes and discovers the actual path MTU
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2. Graceful fallback when datagrams exceed discovered MTU
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3. Expose MTU in metrics for debugging
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## Implementation
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### Phase 1: Enable PMTUD in Quinn
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`crates/wzp-transport/src/config.rs` — update `transport_config()`:
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```rust
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// Enable PMTUD (Quinn default is enabled, but we should ensure it)
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config.mtu_discovery_config(Some(quinn::MtuDiscoveryConfig::default()));
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// Set minimum MTU for safety (some paths can't handle 1200)
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// Quinn default min is 1200, which is the QUIC spec minimum
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```
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Quinn's `MtuDiscoveryConfig` has:
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- `interval`: how often to probe (default: 600s)
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- `upper_bound`: max MTU to probe (default: 1452 for IPv4)
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- `minimum_change`: min MTU increase to be worth probing (default: 20)
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### Phase 2: Handle MTU-related Failures
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In federation forwarding (`send_raw_datagram`), if the datagram exceeds the connection's current MTU, Quinn returns an error. Handle gracefully:
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- Log warning with packet size vs MTU
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- Drop the packet (don't crash)
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- Track in metrics: `wzp_relay_mtu_exceeded_total`
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### Phase 3: Codec-Aware MTU
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When the path MTU is small, the relay or client should:
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- Prefer lower-bitrate codecs (smaller packets)
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- Reduce FEC ratio (fewer repair packets)
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- This feeds into the adaptive quality system
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### Phase 4: Expose MTU in Stats
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- Add `path_mtu` to relay metrics (per peer)
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- Add `path_mtu` to client stats (visible in UI)
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- Log MTU on connection establishment
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## Non-Goals (v1)
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- Datagram fragmentation (QUIC datagrams are atomic — either fit or don't)
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- Manual MTU override per relay config
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- MTU-based codec selection (future, needs adaptive quality)
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## Effort: 1 day
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## Implementation Status (2026-04-12)
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Phase 1 is now implemented:
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### What was built
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- **Transport config** (`crates/wzp-transport/src/config.rs`):
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- `MtuDiscoveryConfig` with `upper_bound=1452`, `interval=300s`, `black_hole_cooldown=30s`
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- `initial_mtu=1200` (safe QUIC minimum)
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- Quinn's PLPMTUD binary-searches from 1200 up to 1452 automatically
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- **`QuinnPathSnapshot::current_mtu`** (`crates/wzp-transport/src/quic.rs`):
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- Reads `connection.max_datagram_size()` which reflects the PMTUD-discovered value
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- Available to all callers via `transport.quinn_path_stats()`
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- **Trunk batcher MTU-aware** (`crates/wzp-relay/src/room.rs`):
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- `TrunkedForwarder::new()` initializes `max_bytes` from discovered MTU
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- `send()` refreshes `max_bytes` on every call (cheap atomic read in quinn)
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- Federation trunk frames grow automatically as PMTUD discovers larger paths
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### Phases 2-3 status
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- Phase 2 (handle MTU failures): Already handled — `send_media()`/`send_trunk()` check `max_datagram_size()` and return `DatagramTooLarge` errors. These are logged and the packet is dropped gracefully.
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- Phase 3 (codec-aware MTU): Not yet implemented. Future video frames will need application-layer fragmentation when they exceed the discovered MTU.
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