# T2.2 — `BandwidthEstimator` in `wzp-proto::bandwidth`

**Status:** Pending Review
**Agent:** Kimi Code CLI
**Started:** 2026-05-11T17:05Z
**Completed:** 2026-05-11T17:12Z
**Commit:** (see git log)
**PRD:** ../PRD-transport-feedback-bwe.md

## What I changed

- `crates/wzp-transport/src/quic.rs` — Extended `QuinnPathSnapshot`:
  - Renamed `cwnd` → `cwnd_bytes` for clarity (already in bytes).
  - Added `bytes_in_flight: u64` (set to 0 because quinn 0.11.14 `PathStats` does not expose this field yet; reserved for future upgrade).
- `crates/wzp-proto/src/bandwidth.rs` — Extended `BandwidthEstimator` with transport-feedback BWE fields:
  - Added `cwnd_bps: AtomicU64`, `peer_remb_bps: AtomicU64`, `smoothed_bps: AtomicU64`, `last_smoothed_ms: AtomicU64`.
  - Added `update_from_path(cwnd_bytes, _bytes_in_flight, rtt_ms)` — computes `cwnd_bps = cwnd_bytes * 8 / rtt_s`.
  - Added `update_from_peer(fb_remb_bps: u32)` — stores peer REMB.
  - Added `target_send_bps(&self) -> u64` — returns `0.9 * min(cwnd_bps, peer_remb_bps)`.
  - Added `smoothed_bps(&self) -> u64` — returns the EWMA-smoothed estimate.
  - EWMA smoothing uses a 2-second half-life: `alpha = 1 - 0.5^(dt_ms / 2000)`.

## Why these choices

`QuinnPathSnapshot` lives in `wzp-transport`; `BandwidthEstimator` lives in `wzp-proto`. Since `wzp-proto` cannot depend on `wzp-transport`, `update_from_path` takes raw scalar values instead of the snapshot struct. Callers in `wzp-client` (T2.3) will destructure `QuinnPathSnapshot` and pass the fields through.

`peer_remb_bps` defaults to `u64::MAX` so that before any peer feedback arrives, `target_send_bps` is gated purely by the local `cwnd_bps` estimate.

## Deviations from the task spec

- Task step 3 shows `update_from_quinn(&self, snap: &QuinnPathSnapshot)`. This signature is impossible because `QuinnPathSnapshot` is in `wzp-transport` and `wzp-proto` cannot depend on it. Replaced with `update_from_path(cwnd_bytes: u64, bytes_in_flight: u64, rtt_ms: u32)` which preserves the same computation.
- `bytes_in_flight` is hard-coded to `0` in `QuinnPathSnapshot` because quinn 0.11.14 does not expose it on `PathStats`. A comment documents this.

## Verification output

```bash
$ cargo test -p wzp-proto bandwidth
running 15 tests
...(all 15 pass)...

test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 103 filtered out; finished in 0.11s
```

```bash
$ cargo test -p wzp-transport
running 11 tests
...(all 11 pass)...

test result: ok. 11 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
```

## Test summary

- Tests added: 3
  - `target_send_bps_uses_min_of_cwnd_and_remb`
  - `target_send_bps_with_zero_cwnd_uses_remb`
  - `smoothed_bps_ewma_converges`
- Tests modified: 0
- `wzp-proto` test count: 115 (was 112 before Wave 2)
- `wzp-transport` test count: 11 (unchanged)
- `cargo clippy -p wzp-proto -p wzp-transport --all-targets -- -D warnings`: pass
- `cargo fmt --all -- --check`: pass

## Risks / follow-ups

- `bytes_in_flight` is stubbed at 0. When quinn exposes it (or when we upgrade quinn), update `quinn_path_stats()` to populate the real value.
- T2.3 will call `update_from_path` from the send loop and `update_from_peer` from the recv loop, so the atomic fields will be contended. `Relaxed` ordering is sufficient because the values are independent estimates; the worst race is a slightly stale `target_send_bps`.

## Reviewer checklist (filled in by reviewer)

- [ ] Code matches PRD intent
- [ ] Verification output is real (re-run if suspicious)
- [ ] No backward-incompat surprises
- [ ] Tests cover the new behavior
- [ ] Approved