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

$ 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

$ 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

3.5 KiB Raw Blame History

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