# PRD: Peer-to-Peer Direct Calls (No Relay)

## Problem

All calls currently route through a relay, even 1-on-1 calls between clients that could reach each other directly. This adds latency (2x hop), creates a single point of failure, and requires trusting the relay operator (even though media is encrypted, the relay sees metadata).

## Solution

For 1-on-1 calls, clients attempt a direct QUIC connection using STUN-discovered addresses. If NAT traversal succeeds, media flows directly between peers. If it fails, fall back to relay-assisted mode (current behavior).

## Architecture

```
Preferred (P2P):
  Client A ←──QUIC direct──→ Client B
  (no relay in media path, true E2E)

Fallback (Relay):
  Client A ──→ Relay ──→ Client B
  (current model)

Hybrid discovery:
  Client A → Relay (signaling only) → Client B
       ↓                                    ↓
    STUN server                        STUN server
       ↓                                    ↓
    Discover public IP:port          Discover public IP:port
       ↓                                    ↓
    Exchange candidates via relay signaling
       ↓                                    ↓
    Attempt direct QUIC connection ←──→
```

## Why P2P = True E2E

- QUIC TLS handshake establishes encrypted tunnel directly between A and B
- No third party sees the traffic
- Certificate pinning via identity fingerprints: each client derives their TLS cert from their Ed25519 seed (same as relay identity). During QUIC handshake, both sides verify the peer's cert fingerprint against the known identity
- MITM elimination: if A knows B's fingerprint (from prior call, QR code, or identity server), any interceptor presents a different cert → fingerprint mismatch → connection rejected
- Stronger guarantee than relay-assisted: user doesn't need to trust relay operator

## Requirements

### Phase 1: STUN Discovery

1. **STUN client**: lightweight UDP-based STUN client to discover public IP:port
   - Use existing public STUN servers (stun.l.google.com:19302, etc.)
   - Or run a STUN server alongside the relay
   - Discover: local addresses, server-reflexive addresses (STUN), relay candidates (TURN/relay fallback)

2. **Candidate gathering**: on call initiation, gather all candidates:
   - Host candidates: local network interfaces
   - Server-reflexive: STUN-discovered public IP:port
   - Relay candidate: the relay's address (fallback)

3. **Candidate exchange**: via relay signaling channel (existing `IceCandidate` signal message)
   - A sends candidates to relay → relay forwards to B
   - B sends candidates to relay → relay forwards to A

### Phase 2: Direct Connection

1. **QUIC hole punching**: both clients simultaneously attempt QUIC connections to each other's candidates
   - Quinn supports connecting to multiple addresses
   - First successful connection wins
   - Timeout after 3 seconds, fall back to relay

2. **Identity verification**: during QUIC handshake, verify peer's TLS cert fingerprint
   - `server_config_from_seed()` already exists — derive client cert from identity seed
   - Both sides present certs (mutual TLS)
   - Verify fingerprint matches expected identity

3. **Media flow**: once connected, use existing `QuinnTransport` for media + signals
   - Same `send_media()` / `recv_media()` API
   - Same codec pipeline, FEC, jitter buffer
   - No code changes needed in the call engine

### Phase 3: Adaptive Quality (P2P)

P2P connections have direct quality visibility — no relay middleman:

1. Both clients observe RTT, loss, jitter directly from QUIC stats
2. Adapt codec quality based on direct observations
3. Since only 2 participants, coordinated switching is simple: propose → ack → switch

This is the simplest case for adaptive quality. Once proven, backport the logic to relay-assisted mode.

### Phase 4: Hybrid Mode

1. **Call initiation**: always connect to relay for signaling
2. **Parallel attempt**: while relay call is active, attempt P2P in background
3. **Seamless migration**: if P2P succeeds, migrate media path from relay to direct
   - Both clients switch simultaneously
   - Relay connection kept alive for signaling (presence, room updates)
4. **Fallback**: if P2P connection drops, seamlessly fall back to relay

## Security Properties

| Property | Relay Mode | P2P Mode |
|----------|-----------|----------|
| Encryption | ChaCha20-Poly1305 (app layer) | QUIC TLS 1.3 + ChaCha20-Poly1305 |
| Key exchange | Via relay signaling | Direct QUIC handshake |
| Identity verification | TOFU (server fingerprint) | Mutual TLS cert pinning |
| Metadata privacy | Relay sees who talks to whom | No third party sees anything |
| MITM resistance | Depends on relay trust | Strong (cert pinning) |
| Forward secrecy | ECDH ephemeral keys | QUIC built-in + app-layer rekey |

## Implementation Notes

### STUN in Rust

Use `stun-rs` or `webrtc-rs` crate for STUN client. Minimal: just need Binding Request/Response to discover server-reflexive address.

### Quinn Hole Punching

Quinn's `Endpoint` can both listen and connect. For hole punching:
```rust
let endpoint = create_endpoint(bind_addr, Some(server_config))?;
// Send connect to peer's address (opens NAT pinhole)
let conn = connect(&endpoint, peer_addr, "peer", client_config).await?;
// Simultaneously, peer connects to our address
// First successful handshake wins
```

### Client TLS Certificate

Already have `server_config_from_seed()` for relays. Create `client_config_from_seed()` that presents a TLS client certificate derived from the identity seed. The peer verifies this cert's fingerprint.

### Signaling via Relay

The existing relay connection carries `IceCandidate` signals. No new infrastructure needed — just use the relay as a dumb signaling pipe for candidate exchange.

## Non-Goals (v1)

- SFU over P2P (P2P is 1-on-1 only; multi-party uses relay SFU)
- TURN server (relay acts as the fallback, no separate TURN)
- mDNS local discovery (future)
- Mesh P2P for multi-party (future, complex)

## Milestones

| Phase | Scope | Effort | Status |
|-------|-------|--------|--------|
| 1 | STUN client + candidate gathering | 2 days | Done |
| 2 | QUIC hole punching + identity verification | 3 days | Done |
| 3 | Adaptive quality on P2P connection | 2 days | Done (#23) |
| 4 | Hybrid mode (relay + P2P, seamless migration) | 3 days | Done |
| 5 | Single-socket Nebula (shared signal+direct endpoint) | 2 days | Done |
| 6 | ICE path negotiation + dual-path race | 3 days | Done |
| 7 | IPv6 dual-socket | 2 days | Done (but `dual_path.rs` integration tests broken — missing `ipv6_endpoint` arg) |
| 8.1 | Public STUN client (RFC 5389) | 1 day | Done |
| 8.2 | PCP/PMP/UPnP port mapping | 2 days | Done |
| 8.3 | Mid-call ICE re-gathering + CandidateUpdate signal | 2 days | Done (signal plane; transport hot-swap TODO) |
| 8.4 | Netcheck diagnostic | 1 day | Done |
| 8.5 | Region-based relay selection (data model) | 1 day | Done |
| 8.6a | Hard NAT: port allocation detection | 1 day | Done |
| 8.6b | Hard NAT: sequential port prediction signal | 1 day | Done (signal + prediction fn; dial integration pending) |
| 8.6c | Hard NAT: birthday attack (256×1024 probes) | 3 days | Not started |
| 8.6d | Hard NAT: hybrid waterfall + background upgrade | 2 days | Not started |

## Implementation Status (2026-04-13)

Phases 1-2, 4-7 are implemented. First P2P call completed 2026-04-12.

### Known regression

Phase 7 added `ipv6_endpoint: Option<Endpoint>` parameter to `race()` in `crates/wzp-client/src/dual_path.rs` but the 3 test call sites in `crates/wzp-client/tests/dual_path.rs` (lines 111, 153, 191) were not updated — they pass 6 args instead of 7. Fix: add `None,` after the `shared_endpoint` arg in each call.

## Update (2026-04-13)

P2P adaptive quality (#23) now implemented:
- Both peers self-observe network quality from QUIC path stats
- Quality reports generated every ~1s and attached to outgoing packets
- AdaptiveQualityController drives codec switching on both P2P and relay calls

## Update (2026-04-14): Phase 8 — Tailscale-Inspired Enhancements

Added 5 new modules to bring NAT traversal capability close to Tailscale's:

### Phase 8.1: Public STUN Client (Done)
- `stun.rs`: RFC 5389 Binding Request/Response over raw UDP
- Independent reflexive discovery via public STUN servers (Google, Cloudflare)
- `detect_nat_type_with_stun()` combines relay + STUN probes for higher confidence
- STUN fallback in desktop's `try_reflect_own_addr()` when relay reflection fails

### Phase 8.2: PCP/PMP/UPnP Port Mapping (Done)
- `portmap.rs`: NAT-PMP (RFC 6886), PCP (RFC 6887), UPnP IGD
- Gateway discovery (macOS + Linux), try NAT-PMP → PCP → UPnP in sequence
- New candidate type: `PeerCandidates.mapped` + signal fields `caller_mapped_addr`/`callee_mapped_addr`/`peer_mapped_addr`
- Dial order: host → mapped → reflexive (mapped helps on symmetric NATs)

### Phase 8.3: Mid-Call ICE Re-Gathering (Done — signal plane)
- `ice_agent.rs`: `IceAgent` with `gather()`, `re_gather()`, `apply_peer_update()`
- `SignalMessage::CandidateUpdate` with monotonic generation counter
- Relay forwards `CandidateUpdate` like `MediaPathReport`
- Desktop handles and emits to JS frontend
- Transport hot-swap: designed but not yet wired into live call engine

### Phase 8.4: Netcheck Diagnostic (Done)
- `netcheck.rs`: comprehensive network diagnostic (NAT type, reflexive addr, IPv4/v6, port mapping, relay latencies)
- CLI: `wzp-client --netcheck <relay>`

### Phase 8.5: Region-Based Relay Selection (Done — data model)
- `relay_map.rs`: `RelayMap` sorted by RTT with `preferred()` selection
- `RegisterPresenceAck` extended with `relay_region` + `available_relays`

### Phase 8.6: Hard NAT Traversal (Phase A done, B-D pending)
- **Phase A (Done)**: Port allocation pattern detection — `PortAllocation` enum (`PortPreserving`/`Sequential{delta}`/`Random`/`Unknown`), `detect_port_allocation()` probes N STUN servers from single socket, `classify_port_allocation()` with wraparound + jitter tolerance, `predict_ports()` for sequential NATs
- **Phase B (signal ready)**: `HardNatProbe` signal message carries `port_sequence`, `allocation`, `external_ip` — relay forwarding implemented. Actual dial-to-predicted-ports integration into `dual_path::race()` pending.
- **Phase C (not started)**: Birthday attack (256 sockets × 1024 probes) for random NATs
- **Phase D (not started)**: Hybrid waterfall with background relay-to-direct upgrade
- `NetcheckReport.port_allocation` populated automatically from `detect_port_allocation()`
- See `docs/PRD-hard-nat.md` for full design
