# T2.5 — Tier B (packet-rate) + Tier C (timestamp drift)

**Status:** Pending Review
**Agent:** Kimi Code CLI
**Started:** 2026-05-11T17:35Z
**Completed:** 2026-05-11T17:45Z
**Commit:** 846c98e
**PRD:** ../PRD-relay-conformance.md

## What I changed

- `crates/wzp-relay/src/conformance.rs` — Extended `ConformanceMeter`:
  - Added `max_pps(codec: CodecId) -> u32`: `1000 / frame_duration_ms * 3`.
  - Tier B check in `observe()`: `packets_in_window > max_pps * 1.5` → `PacketRateExceeded`.
  - Added rolling 200-packet `VecDeque<(seq, timestamp)>` for drift tracking.
  - Tier C check: computes `Δtimestamp / Δseq` over the window; if outside `frame_duration_ms × [0.5, 2.0]`, returns `TimestampDrift`.
  - Handles `u32` wraparound via `wrapping_sub`.

## Why these choices

The `* 3` factor on packet rate mirrors the FEC overhead used in Tier A's bitrate ceiling. The 1.5× multiplier on `max_pps` provides headroom for burstiness.

For timestamp drift, a 200-packet window (~4-8 seconds of audio) gives a stable average while still reacting within a reasonable timeframe. The `[0.5, 2.0]` bounds catch both timestamp acceleration (cheating/fast-forward) and deceleration (stalling/replay).

## Deviations from the task spec

None.

## Verification output

```bash
$ cargo test -p wzp-relay conformance
running 10 tests
test conformance::tests::bitrate_exceeded_for_opus24k ... ok
test conformance::tests::ceiling_bps_floor ... ok
test conformance::tests::packet_rate_exceeded ... ok
test conformance::tests::packet_rate_within_limit ... ok
test conformance::tests::small_packets_stay_within_ceiling ... ok
test conformance::tests::timestamp_drift_detected_when_too_fast ... ok
test conformance::tests::timestamp_drift_detected_when_too_slow ... ok
test conformance::tests::timestamp_drift_not_checked_before_two_packets ... ok
test conformance::tests::timestamp_normal_no_drift ... ok
test conformance::tests::window_resets_after_one_second ... ok

test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured; 76 filtered out; finished in 0.00s
```

## Test summary

- Tests added: 6
  - `packet_rate_exceeded` — 226 packets at Opus24k threshold trips `PacketRateExceeded`
  - `packet_rate_within_limit` — 112 packets at Opus6k threshold stays within limit
  - `timestamp_drift_detected_when_too_fast` — 5ms/packet (below 10ms min) triggers drift
  - `timestamp_drift_detected_when_too_slow` — 50ms/packet (above 40ms max) triggers drift
  - `timestamp_normal_no_drift` — 200 packets at exactly 20ms/packet all pass
  - `timestamp_drift_not_checked_before_two_packets` — single packet never triggers
- Tests modified: 0
- `wzp-relay` test count: 86 (unchanged from T2.4; conformance tests expanded from 4 to 10)
- `cargo clippy -p wzp-relay --lib`: pass
- `cargo fmt --all -- --check`: pass

## Risks / follow-ups

- Timestamp drift uses `u32` wrapping arithmetic. In practice, timestamps wrap after ~49 days of session uptime — the 200-packet window makes wraparound extremely unlikely, but the code handles it correctly.

## Reviewer checklist (filled in by reviewer)

- [ ] Code matches PRD intent
- [ ] Verification output is real
- [ ] No backward-incompat surprises
- [ ] Tests cover the new behavior
- [ ] Approved
