wgengine/magicsock: add probed MTU metrics

Record the number of MTU probes sent, the total bytes sent, the number of times
we got a successful return from an MTU probe of a particular size, and the max
MTU recorded.

Updates #311

Signed-off-by: Val <valerie@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -633,6 +633,12 @@ func (de *endpoint) sendDiscoPing(ep netip.AddrPort, discoKey key.DiscoPublic, t
 	}, logLevel)
 	if !sent {
 		de.forgetDiscoPing(txid)
+		return
+	}
+
+	if size != 0 {
+		metricSentDiscoPeerMTUProbes.Add(1)
+		metricSentDiscoPeerMTUProbeBytes.Add(int64(pingSizeToPktLen(size, ep.Addr().Is6())))
 	}
 }
 
@@ -1062,6 +1068,15 @@ func (de *endpoint) handlePongConnLocked(m *disco.Pong, di *discoInfo, src netip
 	knownTxID = true // for naked returns below
 	de.removeSentDiscoPingLocked(m.TxID, sp)
 
+	pktLen := int(pingSizeToPktLen(sp.size, sp.to.Addr().Is6()))
+	if sp.size != 0 {
+		m := getPeerMTUsProbedMetric(tstun.WireMTU(pktLen))
+		m.Add(1)
+		if metricMaxPeerMTUProbed.Value() < int64(pktLen) {
+			metricMaxPeerMTUProbed.Set(int64(pktLen))
+		}
+	}
+
 	now := mono.Now()
 	latency := now.Sub(sp.at)
 
@@ -1083,7 +1098,7 @@ func (de *endpoint) handlePongConnLocked(m *disco.Pong, di *discoInfo, src netip
 	}
 
 	if sp.purpose != pingHeartbeat {
-		de.c.dlogf("[v1] magicsock: disco: %v<-%v (%v, %v)  got pong tx=%x latency=%v pktlen=%v pong.src=%v%v", de.c.discoShort, de.discoShort(), de.publicKey.ShortString(), src, m.TxID[:6], latency.Round(time.Millisecond), pingSizeToPktLen(sp.size, sp.to.Addr().Is6()), m.Src, logger.ArgWriter(func(bw *bufio.Writer) {
+		de.c.dlogf("[v1] magicsock: disco: %v<-%v (%v, %v)  got pong tx=%x latency=%v pktlen=%v pong.src=%v%v", de.c.discoShort, de.discoShort(), de.publicKey.ShortString(), src, m.TxID[:6], latency.Round(time.Millisecond), pktLen, m.Src, logger.ArgWriter(func(bw *bufio.Writer) {
 			if sp.to != src {
 				fmt.Fprintf(bw, " ping.to=%v", sp.to)
 			}

wgengine/magicsock/magicsock.go

@@ -42,6 +42,7 @@ import (
 	"tailscale.com/net/portmapper"
 	"tailscale.com/net/sockstats"
 	"tailscale.com/net/stun"
+	"tailscale.com/net/tstun"
 	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstime"
@@ -2831,6 +2832,8 @@ var (
 	metricSentDiscoDERP              = clientmetric.NewCounter("magicsock_disco_sent_derp")
 	metricSentDiscoPing              = clientmetric.NewCounter("magicsock_disco_sent_ping")
 	metricSentDiscoPong              = clientmetric.NewCounter("magicsock_disco_sent_pong")
+	metricSentDiscoPeerMTUProbes     = clientmetric.NewCounter("magicsock_disco_sent_peer_mtu_probes")
+	metricSentDiscoPeerMTUProbeBytes = clientmetric.NewCounter("magicsock_disco_sent_peer_mtu_probe_bytes")
 	metricSentDiscoCallMeMaybe       = clientmetric.NewCounter("magicsock_disco_sent_callmemaybe")
 	metricRecvDiscoBadPeer           = clientmetric.NewCounter("magicsock_disco_recv_bad_peer")
 	metricRecvDiscoBadKey            = clientmetric.NewCounter("magicsock_disco_recv_bad_key")
@@ -2852,4 +2855,18 @@ var (
 	// Disco packets received bpf read path
 	metricRecvDiscoPacketIPv4 = clientmetric.NewCounter("magicsock_disco_recv_bpf_ipv4")
 	metricRecvDiscoPacketIPv6 = clientmetric.NewCounter("magicsock_disco_recv_bpf_ipv6")
+
+	// metricMaxPeerMTUProbed is the largest peer path MTU we successfully probed.
+	metricMaxPeerMTUProbed = clientmetric.NewGauge("magicsock_max_peer_mtu_probed")
+
+	// metricRecvDiscoPeerMTUProbesByMTU collects the number of times we
+	// received an peer MTU probe response for a given MTU size.
+	// TODO: add proper support for label maps in clientmetrics
+	metricRecvDiscoPeerMTUProbesByMTU syncs.Map[string, *clientmetric.Metric]
 )
+
+func getPeerMTUsProbedMetric(mtu tstun.WireMTU) *clientmetric.Metric {
+	key := fmt.Sprintf("magicsock_recv_disco_peer_mtu_probes_by_mtu_%d", mtu)
+	mm, _ := metricRecvDiscoPeerMTUProbesByMTU.LoadOrInit(key, func() *clientmetric.Metric { return clientmetric.NewCounter(key) })
+	return mm
+}