diff --git a/crates/wzp-relay/src/config.rs b/crates/wzp-relay/src/config.rs
index d7dc946..b13eea5 100644
--- a/crates/wzp-relay/src/config.rs
+++ b/crates/wzp-relay/src/config.rs
@@ -25,6 +25,10 @@ pub struct RelayConfig {
     /// Port for the Prometheus metrics HTTP endpoint (e.g., 9090).
     /// If None, the metrics endpoint is disabled.
     pub metrics_port: Option<u16>,
+    /// Peer relay addresses to probe for health monitoring.
+    /// Each target gets a persistent QUIC connection sending 1 Ping/s.
+    #[serde(default)]
+    pub probe_targets: Vec<SocketAddr>,
 }
 
 impl Default for RelayConfig {
@@ -38,6 +42,7 @@ impl Default for RelayConfig {
             log_level: "info".to_string(),
             auth_url: None,
             metrics_port: None,
+            probe_targets: Vec::new(),
         }
     }
 }
diff --git a/crates/wzp-relay/src/lib.rs b/crates/wzp-relay/src/lib.rs
index 28f768f..ead05f1 100644
--- a/crates/wzp-relay/src/lib.rs
+++ b/crates/wzp-relay/src/lib.rs
@@ -12,6 +12,7 @@ pub mod config;
 pub mod handshake;
 pub mod metrics;
 pub mod pipeline;
+pub mod probe;
 pub mod room;
 pub mod session_mgr;
 
diff --git a/crates/wzp-relay/src/main.rs b/crates/wzp-relay/src/main.rs
index cc39daf..6365469 100644
--- a/crates/wzp-relay/src/main.rs
+++ b/crates/wzp-relay/src/main.rs
@@ -53,8 +53,16 @@ fn parse_args() -> RelayConfig {
                         .parse().expect("invalid --metrics-port number"),
                 );
             }
+            "--probe" => {
+                i += 1;
+                let addr: SocketAddr = args.get(i)
+                    .expect("--probe requires an address")
+                    .parse()
+                    .expect("invalid --probe address");
+                config.probe_targets.push(addr);
+            }
             "--help" | "-h" => {
-                eprintln!("Usage: wzp-relay [--listen <addr>] [--remote <addr>] [--auth-url <url>] [--metrics-port <port>]");
+                eprintln!("Usage: wzp-relay [--listen <addr>] [--remote <addr>] [--auth-url <url>] [--metrics-port <port>] [--probe <addr>]...");
                 eprintln!();
                 eprintln!("Options:");
                 eprintln!("  --listen <addr>        Listen address (default: 0.0.0.0:4433)");
@@ -62,6 +70,7 @@ fn parse_args() -> RelayConfig {
                 eprintln!("  --auth-url <url>       featherChat auth endpoint (e.g., https://chat.example.com/v1/auth/validate)");
                 eprintln!("                         When set, clients must send a bearer token as first signal message.");
                 eprintln!("  --metrics-port <port>  Prometheus metrics HTTP port (e.g., 9090). Disabled if not set.");
+                eprintln!("  --probe <addr>         Peer relay to probe for health monitoring (repeatable).");
                 eprintln!();
                 eprintln!("Room mode (default):");
                 eprintln!("  Clients join rooms by name. Packets forwarded to all others (SFU).");
@@ -183,6 +192,14 @@ async fn main() -> anyhow::Result<()> {
     // Session manager — enforces max concurrent sessions
     let session_mgr = Arc::new(Mutex::new(SessionManager::new(config.max_sessions)));
 
+    // Spawn inter-relay health probes
+    for target in &config.probe_targets {
+        let probe_config = wzp_relay::probe::ProbeConfig::new(*target);
+        let runner = wzp_relay::probe::ProbeRunner::new(probe_config, metrics.registry());
+        info!(target = %target, "spawning inter-relay health probe");
+        tokio::spawn(async move { runner.run().await });
+    }
+
     if let Some(ref url) = config.auth_url {
         info!(url, "auth enabled — clients must present featherChat token");
     } else {
@@ -217,6 +234,37 @@ async fn main() -> anyhow::Result<()> {
 
             let transport = Arc::new(wzp_transport::QuinnTransport::new(connection));
 
+            // Probe connections use SNI "_probe" to identify themselves.
+            // They skip auth + handshake and just do Ping->Pong.
+            if room_name == "_probe" {
+                info!(%addr, "probe connection detected, entering Ping/Pong responder");
+                loop {
+                    match transport.recv_signal().await {
+                        Ok(Some(wzp_proto::SignalMessage::Ping { timestamp_ms })) => {
+                            if let Err(e) = transport.send_signal(
+                                &wzp_proto::SignalMessage::Pong { timestamp_ms },
+                            ).await {
+                                error!(%addr, "probe pong send error: {e}");
+                                break;
+                            }
+                        }
+                        Ok(Some(_)) => {
+                            // Ignore non-Ping signals on probe connections
+                        }
+                        Ok(None) => {
+                            info!(%addr, "probe connection closed");
+                            break;
+                        }
+                        Err(e) => {
+                            error!(%addr, "probe recv error: {e}");
+                            break;
+                        }
+                    }
+                }
+                transport.close().await.ok();
+                return;
+            }
+
             // Auth check: if --auth-url is set, expect first signal message to be a token
             // Auth: if --auth-url is set, expect AuthToken as first signal
             let authenticated_fp: Option<String> = if let Some(ref url) = auth_url {
@@ -345,15 +393,21 @@ async fn main() -> anyhow::Result<()> {
                     }
                 };
 
+                let session_id_str: String = session_id
+                    .iter()
+                    .map(|b| format!("{b:02x}"))
+                    .collect();
                 room::run_participant(
                     room_mgr.clone(),
                     room_name,
                     participant_id,
                     transport.clone(),
                     metrics.clone(),
+                    &session_id_str,
                 ).await;
 
-                // Participant disconnected — clean up session
+                // Participant disconnected — clean up per-session metrics
+                metrics.remove_session_metrics(&session_id_str);
                 metrics.active_sessions.dec();
                 {
                     let mgr = room_mgr.lock().await;
diff --git a/crates/wzp-relay/src/metrics.rs b/crates/wzp-relay/src/metrics.rs
index 8740c8d..a5674c9 100644
--- a/crates/wzp-relay/src/metrics.rs
+++ b/crates/wzp-relay/src/metrics.rs
@@ -1,9 +1,10 @@
 //! Prometheus metrics for the WZP relay daemon.
 
 use prometheus::{
-    Encoder, Histogram, HistogramOpts, IntCounter, IntCounterVec, IntGauge, Opts, Registry,
-    TextEncoder,
+    Encoder, GaugeVec, Histogram, HistogramOpts, IntCounter, IntCounterVec, IntGauge, IntGaugeVec,
+    Opts, Registry, TextEncoder,
 };
+use wzp_proto::packet::QualityReport;
 use std::sync::Arc;
 
 /// All relay-level Prometheus metrics.
@@ -15,6 +16,12 @@ pub struct RelayMetrics {
     pub bytes_forwarded: IntCounter,
     pub auth_attempts: IntCounterVec,
     pub handshake_duration: Histogram,
+    // Per-session metrics
+    pub session_buffer_depth: IntGaugeVec,
+    pub session_loss_pct: GaugeVec,
+    pub session_rtt_ms: GaugeVec,
+    pub session_underruns: IntCounterVec,
+    pub session_overruns: IntCounterVec,
     registry: Registry,
 }
 
@@ -53,12 +60,58 @@ impl RelayMetrics {
         )
         .expect("metric");
 
+        let session_buffer_depth = IntGaugeVec::new(
+            Opts::new(
+                "wzp_relay_session_jitter_buffer_depth",
+                "Buffer depth per session",
+            ),
+            &["session_id"],
+        )
+        .expect("metric");
+        let session_loss_pct = GaugeVec::new(
+            Opts::new(
+                "wzp_relay_session_loss_pct",
+                "Packet loss percentage per session",
+            ),
+            &["session_id"],
+        )
+        .expect("metric");
+        let session_rtt_ms = GaugeVec::new(
+            Opts::new(
+                "wzp_relay_session_rtt_ms",
+                "Round-trip time per session",
+            ),
+            &["session_id"],
+        )
+        .expect("metric");
+        let session_underruns = IntCounterVec::new(
+            Opts::new(
+                "wzp_relay_session_underruns_total",
+                "Jitter buffer underruns per session",
+            ),
+            &["session_id"],
+        )
+        .expect("metric");
+        let session_overruns = IntCounterVec::new(
+            Opts::new(
+                "wzp_relay_session_overruns_total",
+                "Jitter buffer overruns per session",
+            ),
+            &["session_id"],
+        )
+        .expect("metric");
+
         registry.register(Box::new(active_sessions.clone())).expect("register");
         registry.register(Box::new(active_rooms.clone())).expect("register");
         registry.register(Box::new(packets_forwarded.clone())).expect("register");
         registry.register(Box::new(bytes_forwarded.clone())).expect("register");
         registry.register(Box::new(auth_attempts.clone())).expect("register");
         registry.register(Box::new(handshake_duration.clone())).expect("register");
+        registry.register(Box::new(session_buffer_depth.clone())).expect("register");
+        registry.register(Box::new(session_loss_pct.clone())).expect("register");
+        registry.register(Box::new(session_rtt_ms.clone())).expect("register");
+        registry.register(Box::new(session_underruns.clone())).expect("register");
+        registry.register(Box::new(session_overruns.clone())).expect("register");
 
         Self {
             active_sessions,
@@ -67,10 +120,77 @@ impl RelayMetrics {
             bytes_forwarded,
             auth_attempts,
             handshake_duration,
+            session_buffer_depth,
+            session_loss_pct,
+            session_rtt_ms,
+            session_underruns,
+            session_overruns,
             registry,
         }
     }
 
+    /// Update per-session quality metrics from a QualityReport.
+    pub fn update_session_quality(&self, session_id: &str, report: &QualityReport) {
+        self.session_loss_pct
+            .with_label_values(&[session_id])
+            .set(report.loss_percent() as f64);
+        self.session_rtt_ms
+            .with_label_values(&[session_id])
+            .set(report.rtt_ms() as f64);
+    }
+
+    /// Update per-session buffer metrics.
+    pub fn update_session_buffer(
+        &self,
+        session_id: &str,
+        depth: usize,
+        underruns: u64,
+        overruns: u64,
+    ) {
+        self.session_buffer_depth
+            .with_label_values(&[session_id])
+            .set(depth as i64);
+        // IntCounterVec doesn't have a `set` — we inc by the delta.
+        // Since these are cumulative from the jitter buffer, we use inc_by
+        // with the current totals. To avoid double-counting, callers should
+        // track previous values externally. For simplicity the relay reports
+        // the absolute value each tick; counters only go up so we take the
+        // max(0, new - current) approach.
+        let cur_underruns = self
+            .session_underruns
+            .with_label_values(&[session_id])
+            .get();
+        if underruns > cur_underruns as u64 {
+            self.session_underruns
+                .with_label_values(&[session_id])
+                .inc_by(underruns - cur_underruns as u64);
+        }
+        let cur_overruns = self
+            .session_overruns
+            .with_label_values(&[session_id])
+            .get();
+        if overruns > cur_overruns as u64 {
+            self.session_overruns
+                .with_label_values(&[session_id])
+                .inc_by(overruns - cur_overruns as u64);
+        }
+    }
+
+    /// Remove all per-session label values for a disconnected session.
+    pub fn remove_session_metrics(&self, session_id: &str) {
+        let _ = self.session_buffer_depth.remove_label_values(&[session_id]);
+        let _ = self.session_loss_pct.remove_label_values(&[session_id]);
+        let _ = self.session_rtt_ms.remove_label_values(&[session_id]);
+        let _ = self.session_underruns.remove_label_values(&[session_id]);
+        let _ = self.session_overruns.remove_label_values(&[session_id]);
+    }
+
+    /// Get a reference to the underlying Prometheus registry.
+    /// Probe metrics are registered on this same registry so they appear in /metrics output.
+    pub fn registry(&self) -> &Registry {
+        &self.registry
+    }
+
     /// Gather all metrics and encode them as Prometheus text format.
     pub fn metrics_handler(&self) -> String {
         let encoder = TextEncoder::new();
@@ -123,6 +243,46 @@ mod tests {
         assert!(output.contains("wzp_relay_handshake_duration_seconds"));
     }
 
+    #[test]
+    fn session_quality_update() {
+        let m = RelayMetrics::new();
+        let report = QualityReport {
+            loss_pct: 128,   // ~50%
+            rtt_4ms: 25,     // 100ms
+            jitter_ms: 10,
+            bitrate_cap_kbps: 200,
+        };
+        m.update_session_quality("sess-abc", &report);
+
+        let output = m.metrics_handler();
+        assert!(output.contains("wzp_relay_session_loss_pct{session_id=\"sess-abc\"}"));
+        assert!(output.contains("wzp_relay_session_rtt_ms{session_id=\"sess-abc\"}"));
+        // Verify rtt value (25 * 4 = 100)
+        assert!(output.contains("wzp_relay_session_rtt_ms{session_id=\"sess-abc\"} 100"));
+    }
+
+    #[test]
+    fn session_metrics_cleanup() {
+        let m = RelayMetrics::new();
+        let report = QualityReport {
+            loss_pct: 50,
+            rtt_4ms: 10,
+            jitter_ms: 5,
+            bitrate_cap_kbps: 100,
+        };
+        m.update_session_quality("sess-cleanup", &report);
+        m.update_session_buffer("sess-cleanup", 42, 3, 1);
+
+        // Verify they appear
+        let output = m.metrics_handler();
+        assert!(output.contains("sess-cleanup"));
+
+        // Remove and verify they are gone
+        m.remove_session_metrics("sess-cleanup");
+        let output = m.metrics_handler();
+        assert!(!output.contains("sess-cleanup"));
+    }
+
     #[test]
     fn metrics_increment() {
         let m = RelayMetrics::new();
diff --git a/crates/wzp-relay/src/probe.rs b/crates/wzp-relay/src/probe.rs
new file mode 100644
index 0000000..b62747e
--- /dev/null
+++ b/crates/wzp-relay/src/probe.rs
@@ -0,0 +1,420 @@
+//! Inter-relay health probe.
+//!
+//! A `ProbeRunner` maintains a persistent QUIC connection to a peer relay,
+//! sends 1 Ping/s, and measures RTT, loss, and jitter. Results are exported
+//! as Prometheus gauges with a `target` label.
+
+use std::collections::VecDeque;
+use std::net::SocketAddr;
+use std::sync::Arc;
+use std::time::{Duration, SystemTime, UNIX_EPOCH};
+
+use prometheus::{Gauge, IntGauge, Opts, Registry};
+use tokio::sync::Mutex;
+use tracing::{error, info, warn};
+
+use wzp_proto::{MediaTransport, SignalMessage};
+
+/// Configuration for a single probe target.
+#[derive(Clone, Debug)]
+pub struct ProbeConfig {
+    pub target: SocketAddr,
+    pub interval: Duration,
+}
+
+impl ProbeConfig {
+    pub fn new(target: SocketAddr) -> Self {
+        Self {
+            target,
+            interval: Duration::from_secs(1),
+        }
+    }
+}
+
+/// Prometheus metrics for one probe target.
+pub struct ProbeMetrics {
+    pub rtt_ms: Gauge,
+    pub loss_pct: Gauge,
+    pub jitter_ms: Gauge,
+    pub up: IntGauge,
+}
+
+impl ProbeMetrics {
+    /// Register probe metrics with the given `target` label value.
+    pub fn register(target: &str, registry: &Registry) -> Self {
+        let rtt_ms = Gauge::with_opts(
+            Opts::new("wzp_probe_rtt_ms", "RTT to peer relay in ms")
+                .const_label("target", target),
+        )
+        .expect("probe metric");
+
+        let loss_pct = Gauge::with_opts(
+            Opts::new("wzp_probe_loss_pct", "Packet loss to peer relay in %")
+                .const_label("target", target),
+        )
+        .expect("probe metric");
+
+        let jitter_ms = Gauge::with_opts(
+            Opts::new("wzp_probe_jitter_ms", "Jitter to peer relay in ms")
+                .const_label("target", target),
+        )
+        .expect("probe metric");
+
+        let up = IntGauge::with_opts(
+            Opts::new("wzp_probe_up", "1 if peer relay is reachable, 0 if not")
+                .const_label("target", target),
+        )
+        .expect("probe metric");
+
+        registry.register(Box::new(rtt_ms.clone())).expect("register");
+        registry.register(Box::new(loss_pct.clone())).expect("register");
+        registry.register(Box::new(jitter_ms.clone())).expect("register");
+        registry.register(Box::new(up.clone())).expect("register");
+
+        Self {
+            rtt_ms,
+            loss_pct,
+            jitter_ms,
+            up,
+        }
+    }
+}
+
+/// Sliding window for tracking probe results over the last N pings.
+pub struct SlidingWindow {
+    /// Capacity (number of pings to track).
+    capacity: usize,
+    /// Timestamps of sent pings (ms since epoch) in order.
+    sent: VecDeque<u64>,
+    /// RTT values for received pongs (ms). None = no pong received yet.
+    rtts: VecDeque<Option<f64>>,
+}
+
+impl SlidingWindow {
+    pub fn new(capacity: usize) -> Self {
+        Self {
+            capacity,
+            sent: VecDeque::with_capacity(capacity),
+            rtts: VecDeque::with_capacity(capacity),
+        }
+    }
+
+    /// Record a sent ping.
+    pub fn record_sent(&mut self, timestamp_ms: u64) {
+        if self.sent.len() >= self.capacity {
+            self.sent.pop_front();
+            self.rtts.pop_front();
+        }
+        self.sent.push_back(timestamp_ms);
+        self.rtts.push_back(None);
+    }
+
+    /// Record a received pong. Returns the computed RTT in ms, or None if
+    /// the timestamp doesn't match any pending ping.
+    pub fn record_pong(&mut self, timestamp_ms: u64, now_ms: u64) -> Option<f64> {
+        // Find the sent ping with this timestamp
+        for (i, &sent_ts) in self.sent.iter().enumerate() {
+            if sent_ts == timestamp_ms {
+                let rtt = (now_ms as f64) - (sent_ts as f64);
+                self.rtts[i] = Some(rtt);
+                return Some(rtt);
+            }
+        }
+        None
+    }
+
+    /// Compute loss percentage (0.0-100.0) from the current window.
+    /// A ping is considered lost if it has no matching pong.
+    pub fn loss_pct(&self) -> f64 {
+        if self.sent.is_empty() {
+            return 0.0;
+        }
+        let total = self.rtts.len() as f64;
+        let lost = self.rtts.iter().filter(|r| r.is_none()).count() as f64;
+        (lost / total) * 100.0
+    }
+
+    /// Compute jitter as the standard deviation of RTT values (ms).
+    /// Only considers pings that received a pong.
+    pub fn jitter_ms(&self) -> f64 {
+        let rtts: Vec<f64> = self.rtts.iter().filter_map(|r| *r).collect();
+        if rtts.len() < 2 {
+            return 0.0;
+        }
+        let mean = rtts.iter().sum::<f64>() / rtts.len() as f64;
+        let variance = rtts.iter().map(|r| (r - mean).powi(2)).sum::<f64>() / rtts.len() as f64;
+        variance.sqrt()
+    }
+
+    /// Return the most recent RTT value, if any.
+    pub fn latest_rtt(&self) -> Option<f64> {
+        self.rtts.iter().rev().find_map(|r| *r)
+    }
+}
+
+/// Runs a health probe against a single peer relay.
+pub struct ProbeRunner {
+    config: ProbeConfig,
+    metrics: ProbeMetrics,
+}
+
+impl ProbeRunner {
+    /// Create a new probe runner, registering metrics with the given registry.
+    pub fn new(config: ProbeConfig, registry: &Registry) -> Self {
+        let target_str = config.target.to_string();
+        let metrics = ProbeMetrics::register(&target_str, registry);
+        Self { config, metrics }
+    }
+
+    /// Run the probe forever. This function never returns under normal operation.
+    /// It connects to the target relay, sends Ping every `interval`, and processes
+    /// Pong replies to compute RTT, loss, and jitter.
+    pub async fn run(&self) -> ! {
+        loop {
+            info!(target = %self.config.target, "probe connecting...");
+            match self.run_session().await {
+                Ok(()) => {
+                    // Session ended cleanly (shouldn't happen in practice)
+                    warn!(target = %self.config.target, "probe session ended, reconnecting in 5s");
+                }
+                Err(e) => {
+                    error!(target = %self.config.target, "probe session error: {e}, reconnecting in 5s");
+                }
+            }
+            self.metrics.up.set(0);
+            self.metrics.rtt_ms.set(0.0);
+            tokio::time::sleep(Duration::from_secs(5)).await;
+        }
+    }
+
+    /// Run one probe session (one QUIC connection). Returns when the connection drops.
+    async fn run_session(&self) -> anyhow::Result<()> {
+        // Create a client-only endpoint on an ephemeral port
+        let bind_addr: SocketAddr = "0.0.0.0:0".parse().unwrap();
+        let endpoint = wzp_transport::create_endpoint(bind_addr, None)?;
+        let client_cfg = wzp_transport::client_config();
+        let conn = wzp_transport::connect(
+            &endpoint,
+            self.config.target,
+            "_probe",
+            client_cfg,
+        )
+        .await?;
+
+        let transport = Arc::new(wzp_transport::QuinnTransport::new(conn));
+        self.metrics.up.set(1);
+        info!(target = %self.config.target, "probe connected");
+
+        let window = Arc::new(Mutex::new(SlidingWindow::new(60)));
+
+        // Spawn recv task for pong messages
+        let recv_transport = transport.clone();
+        let recv_window = window.clone();
+        let rtt_gauge = self.metrics.rtt_ms.clone();
+        let loss_gauge = self.metrics.loss_pct.clone();
+        let jitter_gauge = self.metrics.jitter_ms.clone();
+        let up_gauge = self.metrics.up.clone();
+
+        let recv_handle = tokio::spawn(async move {
+            loop {
+                match recv_transport.recv_signal().await {
+                    Ok(Some(SignalMessage::Pong { timestamp_ms })) => {
+                        let now_ms = SystemTime::now()
+                            .duration_since(UNIX_EPOCH)
+                            .unwrap()
+                            .as_millis() as u64;
+                        let mut w = recv_window.lock().await;
+                        if let Some(rtt) = w.record_pong(timestamp_ms, now_ms) {
+                            rtt_gauge.set(rtt);
+                        }
+                        loss_gauge.set(w.loss_pct());
+                        jitter_gauge.set(w.jitter_ms());
+                    }
+                    Ok(Some(_)) => {
+                        // Ignore non-Pong signals
+                    }
+                    Ok(None) => {
+                        info!("probe recv: connection closed");
+                        up_gauge.set(0);
+                        break;
+                    }
+                    Err(e) => {
+                        error!("probe recv error: {e}");
+                        up_gauge.set(0);
+                        break;
+                    }
+                }
+            }
+        });
+
+        // Send ping loop
+        let mut interval = tokio::time::interval(self.config.interval);
+        loop {
+            interval.tick().await;
+
+            if recv_handle.is_finished() {
+                // Recv task died — connection is lost
+                return Ok(());
+            }
+
+            let timestamp_ms = SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .unwrap()
+                .as_millis() as u64;
+
+            {
+                let mut w = window.lock().await;
+                w.record_sent(timestamp_ms);
+            }
+
+            if let Err(e) = transport
+                .send_signal(&SignalMessage::Ping { timestamp_ms })
+                .await
+            {
+                error!(target = %self.config.target, "probe ping send error: {e}");
+                recv_handle.abort();
+                return Err(e.into());
+            }
+        }
+    }
+}
+
+/// Handle an incoming Ping signal by replying with a Pong carrying the same timestamp.
+/// Returns true if the message was a Ping and was handled, false otherwise.
+pub async fn handle_ping(
+    transport: &wzp_transport::QuinnTransport,
+    msg: &SignalMessage,
+) -> bool {
+    if let SignalMessage::Ping { timestamp_ms } = msg {
+        if let Err(e) = transport
+            .send_signal(&SignalMessage::Pong {
+                timestamp_ms: *timestamp_ms,
+            })
+            .await
+        {
+            warn!("failed to send Pong reply: {e}");
+        }
+        true
+    } else {
+        false
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use prometheus::Encoder;
+
+    #[test]
+    fn probe_metrics_register() {
+        let registry = Registry::new();
+        let _metrics = ProbeMetrics::register("127.0.0.1:4433", &registry);
+
+        let encoder = prometheus::TextEncoder::new();
+        let families = registry.gather();
+        let mut buf = Vec::new();
+        encoder.encode(&families, &mut buf).unwrap();
+        let output = String::from_utf8(buf).unwrap();
+
+        assert!(output.contains("wzp_probe_rtt_ms"), "missing wzp_probe_rtt_ms");
+        assert!(output.contains("wzp_probe_loss_pct"), "missing wzp_probe_loss_pct");
+        assert!(output.contains("wzp_probe_jitter_ms"), "missing wzp_probe_jitter_ms");
+        assert!(output.contains("wzp_probe_up"), "missing wzp_probe_up");
+        assert!(
+            output.contains("target=\"127.0.0.1:4433\""),
+            "missing target label"
+        );
+    }
+
+    #[test]
+    fn rtt_calculation() {
+        let mut window = SlidingWindow::new(60);
+
+        // Send a ping at t=1000
+        window.record_sent(1000);
+        // Receive pong at t=1050 => RTT = 50ms
+        let rtt = window.record_pong(1000, 1050);
+        assert_eq!(rtt, Some(50.0));
+
+        // Send at t=2000, receive at t=2030 => RTT = 30ms
+        window.record_sent(2000);
+        let rtt = window.record_pong(2000, 2030);
+        assert_eq!(rtt, Some(30.0));
+
+        assert_eq!(window.latest_rtt(), Some(30.0));
+
+        // Unknown timestamp returns None
+        let rtt = window.record_pong(9999, 10000);
+        assert!(rtt.is_none());
+    }
+
+    #[test]
+    fn loss_calculation() {
+        let mut window = SlidingWindow::new(10);
+
+        // Send 10 pings
+        for i in 0..10 {
+            window.record_sent(i * 1000);
+        }
+
+        // Receive pongs for 7 out of 10 (miss indices 2, 5, 8)
+        for i in 0..10u64 {
+            if i == 2 || i == 5 || i == 8 {
+                continue; // lost
+            }
+            window.record_pong(i * 1000, i * 1000 + 40);
+        }
+
+        // 3 out of 10 lost = 30%
+        let loss = window.loss_pct();
+        assert!((loss - 30.0).abs() < 0.01, "expected ~30%, got {loss}");
+    }
+
+    #[test]
+    fn jitter_calculation() {
+        let mut window = SlidingWindow::new(10);
+
+        // Send 4 pings with known RTTs: 10, 20, 30, 40
+        // Mean = 25, variance = ((15^2 + 5^2 + 5^2 + 15^2) / 4) = (225+25+25+225)/4 = 125
+        // std dev = sqrt(125) ≈ 11.18
+        let rtts = [10.0, 20.0, 30.0, 40.0];
+        for (i, rtt) in rtts.iter().enumerate() {
+            let sent = (i as u64) * 1000;
+            window.record_sent(sent);
+            window.record_pong(sent, sent + *rtt as u64);
+        }
+
+        let jitter = window.jitter_ms();
+        assert!(
+            (jitter - 11.18).abs() < 0.1,
+            "expected jitter ~11.18ms, got {jitter}"
+        );
+    }
+
+    #[test]
+    fn sliding_window_eviction() {
+        let mut window = SlidingWindow::new(5);
+
+        // Fill window
+        for i in 0..5 {
+            window.record_sent(i * 1000);
+        }
+        assert_eq!(window.sent.len(), 5);
+
+        // Add one more — oldest should be evicted
+        window.record_sent(5000);
+        assert_eq!(window.sent.len(), 5);
+        assert_eq!(*window.sent.front().unwrap(), 1000);
+
+        // All 5 are unanswered
+        assert!((window.loss_pct() - 100.0).abs() < 0.01);
+    }
+
+    #[test]
+    fn empty_window_edge_cases() {
+        let window = SlidingWindow::new(60);
+        assert_eq!(window.loss_pct(), 0.0);
+        assert_eq!(window.jitter_ms(), 0.0);
+        assert!(window.latest_rtt().is_none());
+    }
+}
diff --git a/crates/wzp-relay/src/room.rs b/crates/wzp-relay/src/room.rs
index 2ee837a..4ded10a 100644
--- a/crates/wzp-relay/src/room.rs
+++ b/crates/wzp-relay/src/room.rs
@@ -179,6 +179,7 @@ pub async fn run_participant(
     participant_id: ParticipantId,
     transport: Arc<wzp_transport::QuinnTransport>,
     metrics: Arc<RelayMetrics>,
+    session_id: &str,
 ) {
     let addr = transport.connection().remote_address();
     let mut packets_forwarded = 0u64;
@@ -196,6 +197,11 @@ pub async fn run_participant(
             }
         };
 
+        // Update per-session quality metrics if a quality report is present
+        if let Some(ref report) = pkt.quality_report {
+            metrics.update_session_quality(session_id, report);
+        }
+
         // Get current list of other participants
         let others = {
             let mgr = room_mgr.lock().await;