PRD: Transport Feedback & Bandwidth Estimator

Status: proposed Resolves: Audit W6 (no BWE), W14 (no receiver→sender feedback channel). Depends on: PRD #1 (wire format v2 — for u32 seq).

Problem

AdaptiveQualityController decides tier transitions from loss% and RTT only. Quinn exposes congestion-window and bytes-in-flight, but we don't consume them. There is no receiver→sender feedback channel beyond the inline 4-byte QualityReport.

Consequences:

On stable links with spare capacity, we never upgrade past the declared profile (audio stuck at Opus 24 k when 64 k is available).
Oscillation between adjacent tiers on the boundary.
No bandwidth-aware adaptation = no usable video. Video without BWE either oscillates wildly or never uses available capacity.

Goals

Continuous bandwidth estimate per session, surfaced to adaptation controllers.
Receiver→sender feedback at ~50 ms cadence carrying ack/nack/remb.
Audio benefits immediately (smarter upgrades, fewer oscillations).
Video uses BWE as its primary input (PRD #7).

Non-goals

Replacing Quinn's congestion controller — we ride on top.
Cross-stream BWE (each session estimates independently for v1).

Design

`SignalMessage::TransportFeedback`

New signal variant, sent on the existing signal stream every 50 ms or every N media packets, whichever first:

pub struct TransportFeedback {
    pub version: u8,            // PRD #4 W12: always present
    pub stream_id: u8,           // 0 for session-wide; >0 for per-stream
    pub acked_seqs: Vec<u32>,    // recent seqs received OK (RLE-compressed)
    pub nacked_seqs: Vec<u32>,   // recent seqs missing (RLE-compressed)
    pub remb_bps: u32,           // receiver's estimated max bandwidth
    pub recv_time_us: u64,       // arrival-time for sender-side jitter calc
}

RLE compression keeps the wire size bounded (typical payload ~50 B).

`BandwidthEstimator` (in `wzp-proto`)

pub struct BandwidthEstimator {
    cwnd_bps: AtomicU64,         // from Quinn path stats
    bytes_in_flight: AtomicU64,  // from Quinn path stats
    peer_remb_bps: AtomicU64,    // from TransportFeedback
    smoothed_bps: AtomicU64,     // EWMA output
}

impl BandwidthEstimator {
    pub fn update_from_quinn(&self, stats: &QuinnPathStats);
    pub fn update_from_peer(&self, fb: &TransportFeedback);
    pub fn target_send_bps(&self) -> u64 {
        // 0.9 × min(cwnd_bps, peer_remb_bps), EWMA-smoothed
    }
}

Three signals fused:

Quinn cwnd. Conservative ceiling — sending faster than cwnd just drops or queues.
Peer REMB. Receiver's perspective on what they can actually consume (after their own jitter buffer, decode budget, etc.).
EWMA smoothing. Half-life ~2 s; avoids oscillation.

Target = 90 % of min(cwnd, remb), leaving headroom for probing upward.

Adaptation controller integration

AdaptiveQualityController::tick() already consumes loss/RTT/jitter. Add BWE input:

if self.bwe.target_send_bps() > self.current_tier_ceiling_bps() * 1.3
    && consecutive_upgrade_reports >= UPGRADE_THRESHOLD {
    self.upgrade_one_tier();
}

Upgrade gated on BWE headroom, not just clean reports. Eliminates the "always at Opus 24 k on a fiber link" pathology.

Probing

To detect unused capacity, sender occasionally adds 5–10 % padding/FEC during otherwise-clean windows. If cwnd doesn't drop and remb doesn't fall, the headroom is real — upgrade. If signals degrade, back off. Cheap and standard.

Implementation outline

New wzp-proto::bwe::BandwidthEstimator.
wzp-transport exposes QuinnPathStats { cwnd_bps, bytes_in_flight, rtt_ms }; already partially there via QuinnPathSnapshot.
SignalMessage::TransportFeedback variant + serde.
Receiver-side: track recent seqs in a ring buffer; emit feedback every 50 ms.
Sender-side: BWE consumes own Quinn stats + incoming feedback.
AdaptiveQualityController::set_bwe(&BandwidthEstimator).
Prometheus: wzp_session_bwe_bps, wzp_session_remb_bps, wzp_session_cwnd_bps.
Probing logic behind a flag for first deployment.

Acceptance criteria

On a shaped 5 Mbps link with Opus 24 k, controller upgrades to Opus 64 k within 30 s.
On a shaped 50 kbps link, controller stays at Opus 6 k and does not oscillate.
Feedback wire size < 100 B per 50 ms (= < 2 kbps overhead).
Probing finds headroom on a 10 Mbps link in < 60 s.

Risks

Probing-induced loss on already-saturated links. Mitigation: probe only when smoothed loss < 1 % over 10 s.
Feedback storm under heavy loss. Mitigation: feedback rate capped at 20 Hz independent of media rate.
Quinn cwnd lies on QUIC-over-some-VPNs. Mitigation: REMB serves as cross-check; take min of the two.

Effort

~4 engineer-days (Wave 2 tasks T2.1–T2.3).

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