TUN-6699: Add metric for packet too big dropped

quic/datagram.go

@@ -46,8 +46,7 @@ func (dm *DatagramMuxer) mtu() int {
 
 func (dm *DatagramMuxer) SendToSession(session *packet.Session) error {
 	if len(session.Payload) > dm.mtu() {
-		// TODO: TUN-5302 return ICMP packet too big message
-		// drop packet for now, eventually reply with ICMP for PMTUD
+		packetTooBigDropped.Inc()
 		return fmt.Errorf("origin UDP payload has %d bytes, which exceeds transport MTU %d", len(session.Payload), dm.mtu())
 	}
 	payloadWithMetadata, err := suffixSessionID(session.ID, session.Payload)

quic/datagramv2.go

@@ -58,7 +58,7 @@ func NewDatagramMuxerV2(
 // demultiplex the payload from multiple datagram sessions
 func (dm *DatagramMuxerV2) SendToSession(session *packet.Session) error {
 	if len(session.Payload) > dm.mtu() {
-		// TODO: TUN-5302 return ICMP packet too big message
+		packetTooBigDropped.Inc()
 		return fmt.Errorf("origin UDP payload has %d bytes, which exceeds transport MTU %d", len(session.Payload), dm.mtu())
 	}
 	msgWithID, err := suffixSessionID(session.ID, session.Payload)

quic/metrics.go

@@ -144,6 +144,13 @@ var (
 	}
 	registerClient = sync.Once{}
 	registerServer = sync.Once{}
+
+	packetTooBigDropped = prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: namespace,
+		Subsystem: perspectiveString(logging.PerspectiveClient),
+		Name:      "packet_too_big_dropped",
+		Help:      "Count of packets received from origin that are too big to send to the edge and are dropped as a result",
+	})
 )
 
 // MetricsCollector abstracts the difference between client and server metrics from connTracer
@@ -264,6 +271,7 @@ func newClientCollector(index uint8) MetricsCollector {
 			clientMetrics.minRTT,
 			clientMetrics.latestRTT,
 			clientMetrics.smoothedRTT,
+			packetTooBigDropped,
 		)
 	})
 	return &clientCollector{