Classify QUIC stream saturation

agents/rap-node-agent/cmd/rap-node-agent/main.go

@@ -10,6 +10,7 @@ import (
 	"crypto/x509/pkix"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"log"
 	"math/big"
@@ -414,6 +415,7 @@ type vpnFabricSessionDialStats struct {
 	TransportFailures   atomic.Int64
 	SessionOpenFailures atomic.Int64
 	StreamOpenFailures  atomic.Int64
+	CapacityLimited     atomic.Int64
 	AllCandidatesFailed atomic.Int64
 	QUICSelected        atomic.Int64
 	WebSocketSelected   atomic.Int64
@@ -601,6 +603,8 @@ func (s *vpnFabricSessionDialStats) ObserveCandidateFailure(reason string) {
 		s.SessionOpenFailures.Add(1)
 	case "stream_open_failed":
 		s.StreamOpenFailures.Add(1)
+	case "capacity_limited":
+		s.CapacityLimited.Add(1)
 	}
 	s.LastFailureReason.Store(strings.TrimSpace(reason))
 	s.LastFailureUnixSec.Store(time.Now().UTC().Unix())
@@ -653,6 +657,7 @@ func (s *vpnFabricSessionDialStats) Report(observedAt time.Time) map[string]any
 		"transport_failures":     s.TransportFailures.Load(),
 		"session_open_failures":  s.SessionOpenFailures.Load(),
 		"stream_open_failures":   s.StreamOpenFailures.Load(),
+		"capacity_limited":       s.CapacityLimited.Load(),
 		"all_candidates_failed":  s.AllCandidatesFailed.Load(),
 		"quic_selected":          s.QUICSelected.Load(),
 		"websocket_selected":     s.WebSocketSelected.Load(),
@@ -5024,9 +5029,12 @@ func fabricSessionGatewayTransportForAssignment(ctx context.Context, identity st
 		session, err := carrier.Connect(dialCtx, selectedTarget)
 		if err != nil {
 			cancel()
-			meshState.VPNFabricSessionDialStats.ObserveCandidateFailure("session_open_failed")
-			meshState.VPNFabricEndpointObservations.ObserveFailure(selectedTarget.EndpointID, "session_open_failed")
-			log.Printf("vpn fabric session candidate skipped: vpn_connection_id=%s next_hop=%s candidate=%d endpoint=%s transport=%s reason=session_open_failed error=%v", assignment.VPNConnectionID, nextHop, index, selectedTarget.Endpoint, selectedTarget.Transport, err)
+			reason := fabricSessionOpenFailureReason(err)
+			meshState.VPNFabricSessionDialStats.ObserveCandidateFailure(reason)
+			if reason != "capacity_limited" {
+				meshState.VPNFabricEndpointObservations.ObserveFailure(selectedTarget.EndpointID, reason)
+			}
+			log.Printf("vpn fabric session candidate skipped: vpn_connection_id=%s next_hop=%s candidate=%d endpoint=%s transport=%s reason=%s error=%v", assignment.VPNConnectionID, nextHop, index, selectedTarget.Endpoint, selectedTarget.Transport, reason, err)
 			continue
 		}
 		streamID := uint64(time.Now().UnixNano())
@@ -5065,6 +5073,16 @@ func fabricSessionGatewayTransportForAssignment(ctx context.Context, identity st
 	return nil
 }
 
+func fabricSessionOpenFailureReason(err error) string {
+	if err == nil {
+		return ""
+	}
+	if errors.Is(err, mesh.ErrQUICFabricStreamLimitReached) {
+		return "capacity_limited"
+	}
+	return "session_open_failed"
+}
+
 func vpnFabricSessionTarget(meshState *syntheticMeshState, nextHop string) (mesh.FabricTransportTarget, bool) {
 	targets := vpnFabricSessionTargets(meshState, nextHop)
 	if len(targets) == 0 {

agents/rap-node-agent/cmd/rap-node-agent/main_test.go

@@ -5,6 +5,7 @@ import (
 	"crypto/ed25519"
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -775,6 +776,7 @@ func TestHeartbeatPayloadIncludesMeshEndpointReport(t *testing.T) {
 func TestVPNFabricSessionDialStatsReport(t *testing.T) {
 	stats := newVPNFabricSessionDialStats()
 	stats.Attempts.Add(1)
+	stats.ObserveCandidateFailure("capacity_limited")
 	stats.ObserveCandidateFailure("session_open_failed")
 	stats.ObserveSelected(mesh.FabricTransportTarget{
 		Endpoint:       "quic://node-b.example.test:19443",
@@ -785,7 +787,8 @@ func TestVPNFabricSessionDialStatsReport(t *testing.T) {
 	report := stats.Report(time.Date(2026, 5, 16, 12, 0, 0, 0, time.UTC))
 	if report["attempts"] != int64(1) ||
 		report["selected"] != int64(1) ||
-		report["candidate_failures"] != int64(1) ||
+		report["candidate_failures"] != int64(2) ||
+		report["capacity_limited"] != int64(1) ||
 		report["session_open_failures"] != int64(1) ||
 		report["quic_selected"] != int64(1) ||
 		report["pinned_cert_selected"] != int64(1) ||
@@ -796,6 +799,15 @@ func TestVPNFabricSessionDialStatsReport(t *testing.T) {
 	}
 }
 
+func TestFabricSessionOpenFailureReasonClassifiesCapacity(t *testing.T) {
+	if got := fabricSessionOpenFailureReason(mesh.ErrQUICFabricStreamLimitReached); got != "capacity_limited" {
+		t.Fatalf("failure reason = %q, want capacity_limited", got)
+	}
+	if got := fabricSessionOpenFailureReason(errors.New("dial failed")); got != "session_open_failed" {
+		t.Fatalf("failure reason = %q, want session_open_failed", got)
+	}
+}
+
 func TestVPNFabricEndpointObservationReportIsBoundedAndNewestFirst(t *testing.T) {
 	store := newVPNFabricEndpointObservationStore("node-a")
 	base := time.Date(2026, 5, 16, 12, 0, 0, 0, time.UTC)

agents/rap-node-agent/internal/mesh/fabric_quic_transport.go

@@ -18,6 +18,13 @@ import (
 const fabricQUICNextProto = "rap-fabric-data-session-v1"
 const defaultQUICFabricConnIdleTTL = 5 * time.Minute
 const defaultQUICFabricMaxStreamsPerConn = 64
+const ErrQUICFabricStreamLimitReached = quicFabricError("quic fabric stream limit reached")
+
+type quicFabricError string
+
+func (e quicFabricError) Error() string {
+	return string(e)
+}
 
 type QUICFabricTransport struct {
 	Config            *quic.Config
@@ -231,7 +238,7 @@ func (t *QUICFabricTransport) reserveStream(key string, conn *quic.Conn) error {
 	}
 	if entry.activeStreams >= limit {
 		t.stats.StreamLimitRejects++
-		return fmt.Errorf("quic fabric stream limit reached")
+		return ErrQUICFabricStreamLimitReached
 	}
 	entry.activeStreams++
 	entry.lastUsed = time.Now()