diff --git a/crates/wzp-proto/src/bandwidth.rs b/crates/wzp-proto/src/bandwidth.rs
new file mode 100644
index 0000000..166c2e4
--- /dev/null
+++ b/crates/wzp-proto/src/bandwidth.rs
@@ -0,0 +1,454 @@
+//! GCC-style bandwidth estimation and congestion control.
+//!
+//! Tracks available bandwidth using delay-based and loss-based signals,
+//! then adjusts the sending bitrate to avoid congestion. The estimator
+//! uses multiplicative decrease (15%) on congestion and additive increase
+//! (5%) during underuse, following the general shape of Google Congestion
+//! Control (GCC).
+
+use std::collections::VecDeque;
+use std::time::Instant;
+
+use crate::packet::QualityReport;
+use crate::QualityProfile;
+
+/// Network congestion state derived from delay and loss signals.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum CongestionState {
+    /// Network is fine, can increase bandwidth.
+    Underuse,
+    /// Normal operation.
+    Normal,
+    /// Congestion detected, should decrease bandwidth.
+    Overuse,
+}
+
+/// Detects congestion from increasing RTT using an exponential moving average.
+///
+/// Maintains a baseline RTT (minimum observed) and compares the smoothed RTT
+/// against it. If `rtt_ema > baseline * threshold_ratio`, congestion is detected.
+/// The baseline slowly drifts upward to handle route changes.
+struct DelayBasedDetector {
+    /// Baseline RTT (minimum observed).
+    baseline_rtt_ms: f64,
+    /// EMA of recent RTT.
+    rtt_ema: f64,
+    /// EMA smoothing factor.
+    alpha: f64,
+    /// Threshold: if rtt_ema > baseline * threshold_ratio, congestion detected.
+    threshold_ratio: f64,
+    /// Current state.
+    state: CongestionState,
+    /// Whether we have received any RTT sample yet.
+    initialized: bool,
+    /// Drift factor: baseline slowly increases each update to track route changes.
+    baseline_drift: f64,
+}
+
+impl DelayBasedDetector {
+    fn new() -> Self {
+        Self {
+            baseline_rtt_ms: f64::MAX,
+            rtt_ema: 0.0,
+            alpha: 0.3,
+            threshold_ratio: 1.5,
+            state: CongestionState::Normal,
+            initialized: false,
+            baseline_drift: 0.001,
+        }
+    }
+
+    /// Update the detector with a new RTT sample.
+    fn update(&mut self, rtt_ms: f64) {
+        if !self.initialized {
+            self.baseline_rtt_ms = rtt_ms;
+            self.rtt_ema = rtt_ms;
+            self.initialized = true;
+            self.state = CongestionState::Normal;
+            return;
+        }
+
+        // Track minimum RTT as baseline.
+        if rtt_ms < self.baseline_rtt_ms {
+            self.baseline_rtt_ms = rtt_ms;
+        } else {
+            // Slowly drift baseline upward to handle route changes.
+            self.baseline_rtt_ms += self.baseline_drift * (rtt_ms - self.baseline_rtt_ms);
+        }
+
+        // Update EMA.
+        self.rtt_ema = self.alpha * rtt_ms + (1.0 - self.alpha) * self.rtt_ema;
+
+        // Determine state.
+        let overuse_threshold = self.baseline_rtt_ms * self.threshold_ratio;
+        let underuse_threshold = self.baseline_rtt_ms * 1.1;
+
+        if self.rtt_ema > overuse_threshold {
+            self.state = CongestionState::Overuse;
+        } else if self.rtt_ema < underuse_threshold {
+            self.state = CongestionState::Underuse;
+        } else {
+            self.state = CongestionState::Normal;
+        }
+    }
+
+    fn state(&self) -> CongestionState {
+        self.state
+    }
+}
+
+/// Detects congestion from packet loss using a sliding window average.
+struct LossBasedDetector {
+    /// Recent loss percentages (sliding window).
+    loss_window: VecDeque<f64>,
+    /// Maximum window size.
+    window_size: usize,
+    /// Loss threshold for congestion (default 5%).
+    threshold_pct: f64,
+}
+
+impl LossBasedDetector {
+    fn new() -> Self {
+        Self {
+            loss_window: VecDeque::with_capacity(10),
+            window_size: 10,
+            threshold_pct: 5.0,
+        }
+    }
+
+    /// Add a loss percentage sample to the window.
+    fn update(&mut self, loss_pct: f64) {
+        if self.loss_window.len() >= self.window_size {
+            self.loss_window.pop_front();
+        }
+        self.loss_window.push_back(loss_pct);
+    }
+
+    /// Returns true if the average loss in the window exceeds the threshold.
+    fn is_congested(&self) -> bool {
+        if self.loss_window.is_empty() {
+            return false;
+        }
+        let avg = self.loss_window.iter().sum::<f64>() / self.loss_window.len() as f64;
+        avg > self.threshold_pct
+    }
+}
+
+// ─── BandwidthEstimator ─────────────────────────────────────────────────────
+
+/// GCC-style bandwidth estimator that tracks available bandwidth using
+/// delay-based and loss-based congestion signals.
+///
+/// # Algorithm
+///
+/// - **Overuse** (delay or loss): multiplicative decrease by 15%.
+/// - **Underuse** (delay) with no loss congestion: additive increase by 5%.
+/// - **Normal**: hold steady.
+/// - Result is always clamped to `[min_bw_kbps, max_bw_kbps]`.
+pub struct BandwidthEstimator {
+    /// Current estimated bandwidth in kbps.
+    estimated_bw_kbps: f64,
+    /// Minimum bandwidth floor (don't go below this).
+    min_bw_kbps: f64,
+    /// Maximum bandwidth ceiling.
+    max_bw_kbps: f64,
+    /// Delay-based detector state.
+    delay_detector: DelayBasedDetector,
+    /// Loss-based detector state.
+    loss_detector: LossBasedDetector,
+    /// Last update timestamp.
+    last_update: Option<Instant>,
+}
+
+/// Multiplicative decrease factor applied on congestion (15% reduction).
+const DECREASE_FACTOR: f64 = 0.85;
+/// Additive increase factor applied during underuse (5% of current estimate).
+const INCREASE_FACTOR: f64 = 0.05;
+
+impl BandwidthEstimator {
+    /// Create a new bandwidth estimator.
+    ///
+    /// - `initial_bw_kbps`: starting bandwidth estimate.
+    /// - `min`: minimum bandwidth floor in kbps.
+    /// - `max`: maximum bandwidth ceiling in kbps.
+    pub fn new(initial_bw_kbps: f64, min: f64, max: f64) -> Self {
+        Self {
+            estimated_bw_kbps: initial_bw_kbps,
+            min_bw_kbps: min,
+            max_bw_kbps: max,
+            delay_detector: DelayBasedDetector::new(),
+            loss_detector: LossBasedDetector::new(),
+            last_update: None,
+        }
+    }
+
+    /// Update the estimator with new network observations.
+    ///
+    /// Returns the new estimated bandwidth in kbps.
+    ///
+    /// - If delay overuse OR loss congested: decrease by 15% (multiplicative decrease).
+    /// - If delay underuse AND not loss congested: increase by 5% (additive increase).
+    /// - If normal: hold steady.
+    /// - Result is clamped to `[min, max]`.
+    pub fn update(&mut self, rtt_ms: f64, loss_pct: f64, _jitter_ms: f64) -> f64 {
+        self.delay_detector.update(rtt_ms);
+        self.loss_detector.update(loss_pct);
+        self.last_update = Some(Instant::now());
+
+        let delay_state = self.delay_detector.state();
+        let loss_congested = self.loss_detector.is_congested();
+
+        if delay_state == CongestionState::Overuse || loss_congested {
+            // Multiplicative decrease.
+            self.estimated_bw_kbps *= DECREASE_FACTOR;
+        } else if delay_state == CongestionState::Underuse && !loss_congested {
+            // Additive increase.
+            self.estimated_bw_kbps += self.estimated_bw_kbps * INCREASE_FACTOR;
+        }
+        // Normal: hold steady — no change.
+
+        // Clamp to [min, max].
+        self.estimated_bw_kbps = self
+            .estimated_bw_kbps
+            .clamp(self.min_bw_kbps, self.max_bw_kbps);
+
+        self.estimated_bw_kbps
+    }
+
+    /// Current estimated bandwidth in kbps.
+    pub fn estimated_kbps(&self) -> f64 {
+        self.estimated_bw_kbps
+    }
+
+    /// Current congestion state (derived from delay detector).
+    pub fn congestion_state(&self) -> CongestionState {
+        self.delay_detector.state()
+    }
+
+    /// Convenience method: update from a `QualityReport`.
+    ///
+    /// Extracts RTT, loss, and jitter from the report and feeds them into
+    /// the estimator.
+    pub fn from_quality_report(&mut self, report: &QualityReport) -> f64 {
+        let rtt_ms = report.rtt_ms() as f64;
+        let loss_pct = report.loss_percent() as f64;
+        let jitter_ms = report.jitter_ms as f64;
+        self.update(rtt_ms, loss_pct, jitter_ms)
+    }
+
+    /// Recommend a `QualityProfile` based on the current bandwidth estimate.
+    ///
+    /// - bw >= 25 kbps -> GOOD (Opus 24k + 20% FEC = ~28.8 kbps total)
+    /// - bw >= 8 kbps  -> DEGRADED (Opus 6k + 50% FEC = ~9.0 kbps)
+    /// - bw < 8 kbps   -> CATASTROPHIC (Codec2 1.2k + 100% FEC = ~2.4 kbps)
+    pub fn recommended_profile(&self) -> QualityProfile {
+        if self.estimated_bw_kbps >= 25.0 {
+            QualityProfile::GOOD
+        } else if self.estimated_bw_kbps >= 8.0 {
+            QualityProfile::DEGRADED
+        } else {
+            QualityProfile::CATASTROPHIC
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn initial_bandwidth() {
+        let bwe = BandwidthEstimator::new(50.0, 2.0, 100.0);
+        assert!((bwe.estimated_kbps() - 50.0).abs() < f64::EPSILON);
+    }
+
+    #[test]
+    fn stable_network_holds_bandwidth() {
+        let mut bwe = BandwidthEstimator::new(50.0, 2.0, 100.0);
+        // Feed stable, low RTT and 0% loss — after initial sample sets baseline,
+        // subsequent identical RTT should be underuse (rtt_ema < baseline * 1.1),
+        // causing slow increases. The bandwidth should stay near initial or grow slightly.
+        let initial = bwe.estimated_kbps();
+        for _ in 0..20 {
+            bwe.update(30.0, 0.0, 5.0);
+        }
+        // Should not have decreased significantly.
+        assert!(
+            bwe.estimated_kbps() >= initial,
+            "bandwidth should not decrease on stable network: got {} vs initial {}",
+            bwe.estimated_kbps(),
+            initial
+        );
+    }
+
+    #[test]
+    fn high_rtt_decreases_bandwidth() {
+        let mut bwe = BandwidthEstimator::new(50.0, 2.0, 100.0);
+        // Establish a low baseline.
+        for _ in 0..5 {
+            bwe.update(20.0, 0.0, 2.0);
+        }
+        let before = bwe.estimated_kbps();
+
+        // Now feed high RTT to trigger overuse.
+        for _ in 0..10 {
+            bwe.update(200.0, 0.0, 10.0);
+        }
+        assert!(
+            bwe.estimated_kbps() < before,
+            "bandwidth should decrease on high RTT: got {} vs before {}",
+            bwe.estimated_kbps(),
+            before
+        );
+    }
+
+    #[test]
+    fn high_loss_decreases_bandwidth() {
+        let mut bwe = BandwidthEstimator::new(50.0, 2.0, 100.0);
+        let before = bwe.estimated_kbps();
+
+        // Feed 10% loss repeatedly (above the 5% threshold).
+        for _ in 0..15 {
+            bwe.update(20.0, 10.0, 2.0);
+        }
+        assert!(
+            bwe.estimated_kbps() < before,
+            "bandwidth should decrease on high loss: got {} vs before {}",
+            bwe.estimated_kbps(),
+            before
+        );
+    }
+
+    #[test]
+    fn recovery_increases_bandwidth() {
+        let mut bwe = BandwidthEstimator::new(50.0, 2.0, 100.0);
+
+        // Drive bandwidth down with high RTT.
+        for _ in 0..5 {
+            bwe.update(20.0, 0.0, 2.0);
+        }
+        for _ in 0..20 {
+            bwe.update(200.0, 0.0, 10.0);
+        }
+        let low_bw = bwe.estimated_kbps();
+        assert!(low_bw < 50.0, "should have decreased");
+
+        // Now feed good conditions — low RTT should be underuse, causing increase.
+        // Reset the baseline by feeding very low RTT.
+        for _ in 0..30 {
+            bwe.update(10.0, 0.0, 1.0);
+        }
+        assert!(
+            bwe.estimated_kbps() > low_bw,
+            "bandwidth should recover: got {} vs low {}",
+            bwe.estimated_kbps(),
+            low_bw
+        );
+    }
+
+    #[test]
+    fn bandwidth_clamped_to_min() {
+        let mut bwe = BandwidthEstimator::new(10.0, 5.0, 100.0);
+        // Keep feeding congestion to drive bandwidth down.
+        for _ in 0..5 {
+            bwe.update(20.0, 0.0, 2.0);
+        }
+        for _ in 0..100 {
+            bwe.update(500.0, 50.0, 100.0);
+        }
+        assert!(
+            (bwe.estimated_kbps() - 5.0).abs() < f64::EPSILON,
+            "bandwidth should be clamped to min: got {}",
+            bwe.estimated_kbps()
+        );
+    }
+
+    #[test]
+    fn bandwidth_clamped_to_max() {
+        let mut bwe = BandwidthEstimator::new(90.0, 2.0, 100.0);
+        // Keep feeding great conditions to drive bandwidth up.
+        for _ in 0..200 {
+            bwe.update(5.0, 0.0, 1.0);
+        }
+        assert!(
+            bwe.estimated_kbps() <= 100.0,
+            "bandwidth should be clamped to max: got {}",
+            bwe.estimated_kbps()
+        );
+    }
+
+    #[test]
+    fn recommended_profile_thresholds() {
+        // At boundary: >= 25 kbps => GOOD
+        let bwe_good = BandwidthEstimator::new(25.0, 2.0, 100.0);
+        assert_eq!(bwe_good.recommended_profile(), QualityProfile::GOOD);
+
+        // Just below 25 => DEGRADED
+        let bwe_degraded = BandwidthEstimator::new(24.9, 2.0, 100.0);
+        assert_eq!(bwe_degraded.recommended_profile(), QualityProfile::DEGRADED);
+
+        // At boundary: >= 8 kbps => DEGRADED
+        let bwe_degraded2 = BandwidthEstimator::new(8.0, 2.0, 100.0);
+        assert_eq!(
+            bwe_degraded2.recommended_profile(),
+            QualityProfile::DEGRADED
+        );
+
+        // Below 8 => CATASTROPHIC
+        let bwe_cat = BandwidthEstimator::new(7.9, 2.0, 100.0);
+        assert_eq!(
+            bwe_cat.recommended_profile(),
+            QualityProfile::CATASTROPHIC
+        );
+
+        // High bandwidth
+        let bwe_high = BandwidthEstimator::new(80.0, 2.0, 100.0);
+        assert_eq!(bwe_high.recommended_profile(), QualityProfile::GOOD);
+    }
+
+    #[test]
+    fn from_quality_report_integration() {
+        let mut bwe = BandwidthEstimator::new(50.0, 2.0, 100.0);
+
+        // Build a QualityReport with moderate loss and RTT.
+        let report = QualityReport {
+            loss_pct: (10.0_f32 / 100.0 * 255.0) as u8, // ~10% loss
+            rtt_4ms: 25,                                   // 100ms RTT
+            jitter_ms: 10,
+            bitrate_cap_kbps: 200,
+        };
+
+        let new_bw = bwe.from_quality_report(&report);
+        // Should return a valid bandwidth value.
+        assert!(new_bw > 0.0);
+        assert!(new_bw <= 100.0);
+        // The estimator should have been updated.
+        assert!((bwe.estimated_kbps() - new_bw).abs() < f64::EPSILON);
+    }
+
+    // ── Additional detector unit tests ──────────────────────────────────
+
+    #[test]
+    fn delay_detector_starts_normal() {
+        let det = DelayBasedDetector::new();
+        assert_eq!(det.state(), CongestionState::Normal);
+    }
+
+    #[test]
+    fn loss_detector_below_threshold() {
+        let mut det = LossBasedDetector::new();
+        for _ in 0..10 {
+            det.update(2.0); // 2% loss, well below 5% threshold
+        }
+        assert!(!det.is_congested());
+    }
+
+    #[test]
+    fn loss_detector_above_threshold() {
+        let mut det = LossBasedDetector::new();
+        for _ in 0..10 {
+            det.update(8.0); // 8% loss, above 5% threshold
+        }
+        assert!(det.is_congested());
+    }
+}
diff --git a/crates/wzp-proto/src/lib.rs b/crates/wzp-proto/src/lib.rs
index 6719e9b..1d0d59a 100644
--- a/crates/wzp-proto/src/lib.rs
+++ b/crates/wzp-proto/src/lib.rs
@@ -12,6 +12,7 @@
 //! - Identity = 32-byte seed → HKDF → Ed25519 (signing) + X25519 (encryption)
 //! - Fingerprint = SHA-256(Ed25519 pub)[:16]
 
+pub mod bandwidth;
 pub mod codec_id;
 pub mod error;
 pub mod jitter;
@@ -27,6 +28,7 @@ pub use packet::{
     HangupReason, MediaHeader, MediaPacket, MiniFrameContext, MiniHeader, QualityReport,
     SignalMessage, TrunkEntry, TrunkFrame, FRAME_TYPE_FULL, FRAME_TYPE_MINI,
 };
+pub use bandwidth::{BandwidthEstimator, CongestionState};
 pub use quality::{AdaptiveQualityController, Tier};
 pub use session::{Session, SessionEvent, SessionState};
 pub use traits::*;