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

$ 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

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