132 lines
5.1 KiB
Markdown
132 lines
5.1 KiB
Markdown
# WZP Protocol Specification (one-page reference)
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> Distilled from `docs/ARCHITECTURE.md` and the `wzp-proto` crate. Authoritative wire details live in `crates/wzp-proto/src/packet.rs`.
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>
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> **Status:** v1 (audio-only) is the deployed protocol. v2 (audio + video, 16 B header, MediaType, u32 seq, etc.) is specified in `ROAD-TO-VIDEO.md` Phase V1 and supersedes this document when implemented.
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## Layer summary
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| Layer | WZP | FaceTime equivalent |
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|---|---|---|
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| Transport | **QUIC datagrams** (Quinn), PLPMTUD 1200 → 1452 | RTP/SRTP over UDP, ICE |
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| Signaling | `SignalMessage` (bincode) over a QUIC stream, SNI = hashed room name | APNs-tunneled binary plist |
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| Identity | Ed25519 + X25519 from BIP39 seed; fingerprint = SHA-256(pubkey)[..16] | IDS RSA + ECDSA per device |
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| Key agreement | X25519 DH + HKDF, Ed25519 signatures, rekey every 65,536 packets | Per-call DH signed by IDS keys |
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| Bulk crypto | ChaCha20-Poly1305, 64-packet sliding anti-replay | SRTP (AES-CTR + HMAC) |
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| Loss recovery | **RaptorQ FEC + Opus DRED + classical PLC** | NACK / PLI + reference-picture selection |
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| Adaptive | 3-tier hysteresis (Good / Degraded / Catastrophic) + continuous DRED tuner | Per-frame bitrate ladder |
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| Topology | SFU rooms + inter-relay federation + P2P via ICE | Mesh ≤ ~3, SFU above, Apple relays |
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| Header | 12 B `MediaHeader` / 4 B `MiniHeader` (49 of 50), 4 B `QualityReport` trailer | RTP 12 B + extensions |
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## Distinctive choices
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- **QUIC datagrams instead of raw UDP + SRTP.** Brings TLS 1.3, PLPMTUD, path migration, and ACK-based RTT/loss estimation for free.
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- **Continuous DRED tuning.** Maps live `(loss%, RTT, jitter)` to a continuous Opus DRED lookback window. Most stacks treat DRED as discrete tiers.
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- **MiniHeader (4 B for 49/50 packets).** Saves ~8 B/packet ≈ 400 B/s/stream at 50 pps.
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- **E2E-preserving SFU.** The relay forwards encrypted datagrams; it never decrypts media. Room membership uses SNI = `hash(room_name)`.
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- **Codec coordination via `QualityReport` trailer.** Receivers attach 4-byte loss/RTT/jitter/cap to media packets; the SFU broadcasts `QualityDirective` so all senders in a room converge on the same tier.
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## Wire format (current — v1)
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### `MediaHeader` (12 bytes)
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```
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Byte 0: [V:1][T:1][CodecID:4][Q:1][FecRatioHi:1]
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Byte 1: [FecRatioLo:6][unused:2]
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Bytes 2-3: sequence (u16 BE)
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Bytes 4-7: timestamp_ms (u32 BE)
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Byte 8: fec_block_id (u8)
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Byte 9: fec_symbol_idx (u8)
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Byte 10: reserved
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Byte 11: csrc_count
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```
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| Field | Bits | Meaning |
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|---|---|---|
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| V | 1 | Protocol version |
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| T | 1 | 1 = FEC repair packet |
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| CodecID | 4 | See codec table |
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| Q | 1 | QualityReport trailer present |
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| FecRatio | 7 | 0–127 → 0.0–2.0 |
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| sequence | 16 | Wrapping packet seq |
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| timestamp_ms | 32 | ms since session start |
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| fec_block_id | 8 | FEC source block ID |
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| fec_symbol_idx | 8 | Symbol index in block |
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### Codec table
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| ID | Codec | Bitrate | Sample | Frame |
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|---|---|---|---|---|
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| 0 | Opus 24k | 24 kbps | 48 kHz | 20 ms |
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| 1 | Opus 16k | 16 kbps | 48 kHz | 20 ms |
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| 2 | Opus 6k | 6 kbps | 48 kHz | 40 ms |
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| 3 | Codec2 3200 | 3.2 kbps | 8 kHz | 20 ms |
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| 4 | Codec2 1200 | 1.2 kbps | 8 kHz | 40 ms |
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| 5 | ComfortNoise | 0 | 48 kHz | 20 ms |
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| 6 | Opus 32k | 32 kbps | 48 kHz | 20 ms |
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| 7 | Opus 48k | 48 kbps | 48 kHz | 20 ms |
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| 8 | Opus 64k | 64 kbps | 48 kHz | 20 ms |
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### `MiniHeader` (4 bytes, compressed — 49 of every 50 packets)
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```
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[FRAME_TYPE_MINI = 0x01]
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Bytes 0-1: timestamp_delta_ms (u16 BE)
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Bytes 2-3: payload_len (u16 BE)
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```
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Full header sent every 50th packet to resync.
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### `TrunkFrame` (batched, relay-internal)
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```
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[count: u16]
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[session_id: 2][len: u16][payload: len] × count
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```
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Up to 10 entries or PMTUD-discovered MTU; flushed every 5 ms.
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### `QualityReport` (4 bytes, optional inline trailer)
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```
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Byte 0: loss_pct (0-255 → 0-100%)
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Byte 1: rtt_4ms (0-255 → 0-1020 ms)
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Byte 2: jitter_ms (0-255 ms)
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Byte 3: bitrate_cap_kbps (0-255 kbps)
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```
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## Session lifecycle
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```
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Idle → Connecting → Handshaking → Active ⇄ Rekeying → Closed
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```
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- `CallOffer { identity_pub, ephemeral_pub, signature, profiles }`
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- `CallAnswer { identity_pub, ephemeral_pub, signature, chosen_profile }`
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- `session_key = HKDF(X25519_DH(eph_a, eph_b), "warzone-session-key")`
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- Rekey every 65,536 packets via fresh ephemeral DH.
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## SFU forwarding rules
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1. Fan-out to all room participants except the sender.
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2. Failed sends are skipped; forwarding is best-effort.
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3. The relay never decrypts media.
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4. With trunking on, packets to the same receiver are batched (flush 5 ms).
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5. `QualityDirective` is broadcast when the room-wide tier degrades.
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## Adaptive quality (audio, today)
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| Tier | Codec | FEC | Frame |
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|---|---|---|---|
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| Good | Opus 24 k | 20 % | 20 ms |
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| Degraded | Opus 6 k | 50 % | 40 ms |
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| Catastrophic | Codec2 1200 | 100 % | 40 ms |
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Hysteresis: 3 reports to downgrade (2 on cellular), 10 to upgrade.
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## NAT traversal (Phase 8)
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- Candidate types: Host, Port-mapped (NAT-PMP / PCP / UPnP), Server-reflexive (STUN), Relay.
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- Hard-NAT port prediction with `classify_port_allocation()` → `predict_ports()` → `HardNatProbe` signal.
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- Mid-call re-gather: `CandidateUpdate { generation }`.
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