wgengine/magicsock: implement probing of UDP path lifetime (#10844)

This commit implements probing of UDP path lifetime on the tail end of
an active direct connection. Probing configuration has two parts -
Cliffs, which are various timeout cliffs of interest, and
CycleCanStartEvery, which limits how often a probing cycle can start,
per-endpoint. Initially a statically defined default configuration will
be used. The default configuration has cliffs of 10s, 30s, and 60s,
with a CycleCanStartEvery of 24h. Probing results are communicated via
clientmetric counters. Probing is off by default, and can be enabled
via control knob. Probing is purely informational and does not yet
drive any magicsock behaviors.

Updates #540

Signed-off-by: Jordan Whited <jordan@tailscale.com>

control/controlknobs/controlknobs.go

@@ -69,6 +69,10 @@ type Knobs struct {
 	// renewing node keys without breaking connections.
 	// http://go/seamless-key-renewal
 	SeamlessKeyRenewal atomic.Bool
+
+	// ProbeUDPLifetime is whether the node should probe UDP path lifetime on
+	// the tail end of an active direct connection in magicsock.
+	ProbeUDPLifetime atomic.Bool
 }
 
 // UpdateFromNodeAttributes updates k (if non-nil) based on the provided self
@@ -95,6 +99,7 @@ func (k *Knobs) UpdateFromNodeAttributes(selfNodeAttrs []tailcfg.NodeCapability,
 		forceIPTables                 = has(tailcfg.NodeAttrLinuxMustUseIPTables)
 		forceNfTables                 = has(tailcfg.NodeAttrLinuxMustUseNfTables)
 		seamlessKeyRenewal            = has(tailcfg.NodeAttrSeamlessKeyRenewal)
+		probeUDPLifetime              = has(tailcfg.NodeAttrProbeUDPLifetime)
 	)
 
 	if has(tailcfg.NodeAttrOneCGNATEnable) {
@@ -116,6 +121,7 @@ func (k *Knobs) UpdateFromNodeAttributes(selfNodeAttrs []tailcfg.NodeCapability,
 	k.LinuxForceIPTables.Store(forceIPTables)
 	k.LinuxForceNfTables.Store(forceNfTables)
 	k.SeamlessKeyRenewal.Store(seamlessKeyRenewal)
+	k.ProbeUDPLifetime.Store(probeUDPLifetime)
 }
 
 // AsDebugJSON returns k as something that can be marshalled with json.Marshal
@@ -138,5 +144,6 @@ func (k *Knobs) AsDebugJSON() map[string]any {
 		"LinuxForceIPTables":            k.LinuxForceIPTables.Load(),
 		"LinuxForceNfTables":            k.LinuxForceNfTables.Load(),
 		"SeamlessKeyRenewal":            k.SeamlessKeyRenewal.Load(),
+		"ProbeUDPLifetime":              k.ProbeUDPLifetime.Load(),
 	}
 }

ipn/ipnlocal/local.go

@@ -4556,6 +4556,7 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	}
 
 	b.MagicConn().SetSilentDisco(b.ControlKnobs().SilentDisco.Load())
+	b.MagicConn().SetProbeUDPLifetime(b.ControlKnobs().ProbeUDPLifetime.Load())
 
 	b.setDebugLogsByCapabilityLocked(nm)
 

tailcfg/tailcfg.go

@@ -126,7 +126,8 @@ type CapabilityVersion int
 //   - 83: 2023-12-18: Client understands DefaultAutoUpdate
 //   - 84: 2024-01-04: Client understands SeamlessKeyRenewal
 //   - 85: 2024-01-05: Client understands MaxKeyDuration
-const CurrentCapabilityVersion CapabilityVersion = 85
+//   - 86: 2024-01-23: Client understands NodeAttrProbeUDPLifetime
+const CurrentCapabilityVersion CapabilityVersion = 86
 
 type StableID string
 
@@ -2203,6 +2204,10 @@ const (
 	// NodeAttrSeamlessKeyRenewal makes clients enable beta functionality
 	// of renewing node keys without breaking connections.
 	NodeAttrSeamlessKeyRenewal NodeCapability = "seamless-key-renewal"
+
+	// NodeAttrProbeUDPLifetime makes the client probe UDP path lifetime at the
+	// tail end of an active direct connection in magicsock.
+	NodeAttrProbeUDPLifetime NodeCapability = "probe-udp-lifetime"
 )
 
 // SetDNSRequest is a request to add a DNS record.

types/key/disco.go

@@ -4,6 +4,7 @@
 package key
 
 import (
+	"bytes"
 	"crypto/subtle"
 	"fmt"
 
@@ -127,6 +128,14 @@ func (k DiscoPublic) String() string {
 	return string(bs)
 }
 
+// Compare returns an integer comparing DiscoPublic k and l lexicographically.
+// The result will be 0 if k == l, -1 if k < l, and +1 if k > l. This is useful
+// for situations requiring only one node in a pair to perform some operation,
+// e.g. probing UDP path lifetime.
+func (k DiscoPublic) Compare(l DiscoPublic) int {
+	return bytes.Compare(k.k[:], l.k[:])
+}
+
 // AppendText implements encoding.TextAppender.
 func (k DiscoPublic) AppendText(b []byte) ([]byte, error) {
 	return appendHexKey(b, discoPublicHexPrefix, k.k[:]), nil

wgengine/magicsock/debughttp.go

@@ -123,11 +123,11 @@ func (c *Conn) ServeHTTPDebug(w http.ResponseWriter, r *http.Request) {
 }
 
 func printEndpointHTML(w io.Writer, ep *endpoint) {
-	lastRecv := ep.lastRecv.LoadAtomic()
+	lastRecv := ep.lastRecvWG.LoadAtomic()
 
 	ep.mu.Lock()
 	defer ep.mu.Unlock()
-	if ep.lastSend == 0 && lastRecv == 0 {
+	if ep.lastSendExt == 0 && lastRecv == 0 {
 		return // no activity ever
 	}
 
@@ -142,7 +142,7 @@ func printEndpointHTML(w io.Writer, ep *endpoint) {
 
 	fmt.Fprintf(w, "<p>Best: <b>%+v</b>, %v ago (for %v)</p>\n", ep.bestAddr, fmtMono(ep.bestAddrAt), ep.trustBestAddrUntil.Sub(mnow).Round(time.Millisecond))
 	fmt.Fprintf(w, "<p>heartbeating: %v</p>\n", ep.heartBeatTimer != nil)
-	fmt.Fprintf(w, "<p>lastSend: %v ago</p>\n", fmtMono(ep.lastSend))
+	fmt.Fprintf(w, "<p>lastSend: %v ago</p>\n", fmtMono(ep.lastSendExt))
 	fmt.Fprintf(w, "<p>lastFullPing: %v ago</p>\n", fmtMono(ep.lastFullPing))
 
 	eps := make([]netip.AddrPort, 0, len(ep.endpointState))

wgengine/magicsock/derp.go

@@ -26,6 +26,7 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tstime/mono"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/mak"
@@ -668,7 +669,7 @@ func (c *Conn) processDERPReadResult(dm derpReadResult, b []byte) (n int, ep *en
 		return 0, nil
 	}
 
-	ep.noteRecvActivity(ipp)
+	ep.noteRecvActivity(ipp, mono.Now())
 	if stats := c.stats.Load(); stats != nil {
 		stats.UpdateRxPhysical(ep.nodeAddr, ipp, dm.n)
 	}

wgengine/magicsock/discopingpurpose_string.go

@@ -14,11 +14,12 @@ func _() {
 	_ = x[pingDiscovery-0]
 	_ = x[pingHeartbeat-1]
 	_ = x[pingCLI-2]
+	_ = x[pingHeartbeatForUDPLifetime-3]
 }
 
-const _discoPingPurpose_name = "DiscoveryHeartbeatCLI"
+const _discoPingPurpose_name = "DiscoveryHeartbeatCLIHeartbeatForUDPLifetime"
 
-var _discoPingPurpose_index = [...]uint8{0, 9, 18, 21}
+var _discoPingPurpose_index = [...]uint8{0, 9, 18, 21, 44}
 
 func (i discoPingPurpose) String() string {
 	if i < 0 || i >= discoPingPurpose(len(_discoPingPurpose_index)-1) {

wgengine/magicsock/endpoint.go

@@ -15,6 +15,7 @@ import (
 	"net/netip"
 	"reflect"
 	"runtime"
+	"slices"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -55,7 +56,8 @@ func init() {
 // to the peer.
 type endpoint struct {
 	// atomically accessed; declared first for alignment reasons
-	lastRecv              mono.Time
+	lastRecvWG            mono.Time // last time there were incoming packets from this peer destined for wireguard-go (e.g. not disco)
+	lastRecvUDPAny        mono.Time // last time there were incoming UDP packets from this peer of any kind
 	numStopAndResetAtomic int64
 	debugUpdates          *ringbuffer.RingBuffer[EndpointChange]
 
@@ -73,11 +75,12 @@ type endpoint struct {
 	mu sync.Mutex // Lock ordering: Conn.mu, then endpoint.mu
 
 	heartBeatTimer *time.Timer    // nil when idle
-	lastSend       mono.Time      // last time there was outgoing packets sent to this peer (from wireguard-go)
+	lastSendExt    mono.Time      // last time there were outgoing packets sent to this peer from an external trigger (e.g. wireguard-go or disco pingCLI)
+	lastSendAny    mono.Time      // last time there were outgoing packets sent this peer from any trigger, internal or external to magicsock
 	lastFullPing   mono.Time      // last time we pinged all disco or wireguard only endpoints
 	derpAddr       netip.AddrPort // fallback/bootstrap path, if non-zero (non-zero for well-behaved clients)
 
-	bestAddr           addrQuality // best non-DERP path; zero if none
+	bestAddr           addrQuality // best non-DERP path; zero if none; mutate via setBestAddrLocked()
 	bestAddrAt         mono.Time   // time best address re-confirmed
 	trustBestAddrUntil mono.Time   // time when bestAddr expires
 	sentPing           map[stun.TxID]sentPing
@@ -88,11 +91,240 @@ type endpoint struct {
 	// implementation that's a WIP as of 2022-10-20.
 	// See #540 for background.
 	heartbeatDisabled bool
+	probeUDPLifetime  *probeUDPLifetime // UDP path lifetime probing; nil if disabled
 
 	expired         bool // whether the node has expired
 	isWireguardOnly bool // whether the endpoint is WireGuard only
 }
 
+func (de *endpoint) setBestAddrLocked(v addrQuality) {
+	if v.AddrPort != de.bestAddr.AddrPort {
+		de.probeUDPLifetime.resetCycleEndpointLocked()
+	}
+	de.bestAddr = v
+}
+
+const (
+	// udpLifetimeProbeCliffSlack is how much slack to use relative to a
+	// ProbeUDPLifetimeConfig.Cliffs duration in order to account for RTT,
+	// scheduling jitter, buffers, etc. If the cliff is 10s, we attempt to probe
+	// after 10s - 2s (8s) amount of inactivity.
+	udpLifetimeProbeCliffSlack = time.Second * 2
+	// udpLifetimeProbeSchedulingTolerance is how much of a difference can be
+	// tolerated between a UDP lifetime probe scheduling target and when it
+	// actually fired. This must be some fraction of udpLifetimeProbeCliffSlack.
+	udpLifetimeProbeSchedulingTolerance = udpLifetimeProbeCliffSlack / 8
+)
+
+// probeUDPLifetime represents the configuration and state tied to probing UDP
+// path lifetime. A probe "cycle" involves pinging the UDP path at various
+// timeout cliffs, which are pre-defined durations of interest commonly used by
+// NATs/firewalls as default stateful session timeout values. Cliffs are probed
+// in ascending order. A "cycle" completes when all cliffs have received a pong,
+// or when a ping times out. Cycles may extend across endpoint session lifetimes
+// if they are disrupted by user traffic.
+type probeUDPLifetime struct {
+	// All fields are guarded by endpoint.mu. probeUDPLifetime methods are for
+	// convenience.
+
+	// config holds the probing configuration.
+	config ProbeUDPLifetimeConfig
+
+	// timer is nil when idle. A non-nil timer indicates we intend to probe a
+	// timeout cliff in the future.
+	timer *time.Timer
+
+	// bestAddr contains the endpoint.bestAddr.AddrPort at the time a cycle was
+	// scheduled to start. A probing cycle is 1:1 with the current
+	// endpoint.bestAddr.AddrPort in the interest of simplicity. When
+	// endpoint.bestAddr.AddrPort changes, any active probing cycle will reset.
+	bestAddr netip.AddrPort
+	// cycleStartedAt contains the time at which the first cliff
+	// (ProbeUDPLifetimeConfig.Cliffs[0]) was pinged for the current/last cycle.
+	cycleStartedAt time.Time
+	// cycleActive is true if a probing cycle is active, otherwise false.
+	cycleActive bool
+	// currentCliff represents the index into ProbeUDPLifetimeConfig.Cliffs for
+	// the cliff that we are waiting to ping, or waiting on a pong/timeout.
+	currentCliff int
+	// lastTxID is the ID for the last ping that was sent.
+	lastTxID stun.TxID
+}
+
+func (p *probeUDPLifetime) currentCliffDurationEndpointLocked() time.Duration {
+	if p == nil {
+		return 0
+	}
+	return p.config.Cliffs[p.currentCliff]
+}
+
+// cycleCompleteMaxCliffEndpointLocked records the max cliff (as an index of
+// ProbeUDPLifetimeConfig.Cliffs) a probing cycle reached, i.e. received a pong
+// for. A value < 0 indicates no cliff was reached. It is a no-op if the active
+// configuration does not equal defaultProbeUDPLifetimeConfig.
+func (p *probeUDPLifetime) cycleCompleteMaxCliffEndpointLocked(cliff int) {
+	if !p.config.Equals(defaultProbeUDPLifetimeConfig) {
+		return
+	}
+	switch {
+	case cliff < 0:
+		metricUDPLifetimeCycleCompleteNoCliffReached.Add(1)
+	case cliff == 0:
+		metricUDPLifetimeCycleCompleteAt10sCliff.Add(1)
+	case cliff == 1:
+		metricUDPLifetimeCycleCompleteAt30sCliff.Add(1)
+	case cliff == 2:
+		metricUDPLifetimeCycleCompleteAt60sCliff.Add(1)
+	}
+}
+
+// resetCycleEndpointLocked resets the state contained in p to reflect an
+// inactive cycle.
+func (p *probeUDPLifetime) resetCycleEndpointLocked() {
+	if p == nil {
+		return
+	}
+	if p.timer != nil {
+		p.timer.Stop()
+		p.timer = nil
+	}
+	p.cycleActive = false
+	p.currentCliff = 0
+	p.bestAddr = netip.AddrPort{}
+}
+
+// ProbeUDPLifetimeConfig represents the configuration for probing UDP path
+// lifetime.
+type ProbeUDPLifetimeConfig struct {
+	// The timeout cliffs to probe. Values are in ascending order. Ascending
+	// order is chosen over descending because we have limited opportunities to
+	// probe. With a descending order we are stuck waiting for a new UDP
+	// path/session if the first value times out. When that new path is
+	// established is anyone's guess.
+	Cliffs []time.Duration
+	// CycleCanStartEvery represents the min duration between cycles starting
+	// up.
+	CycleCanStartEvery time.Duration
+}
+
+var (
+	// defaultProbeUDPLifetimeConfig is the configuration that must be used
+	// for UDP path lifetime probing until it can be wholly disseminated (not
+	// just on/off) from upstream control components, and associated metrics
+	// (metricUDPLifetime*) have lifetime management.
+	//
+	// TODO(#10928): support dynamic config via tailcfg.PeerCapMap.
+	defaultProbeUDPLifetimeConfig = &ProbeUDPLifetimeConfig{
+		Cliffs: []time.Duration{
+			time.Second * 10,
+			time.Second * 30,
+			time.Second * 60,
+		},
+		CycleCanStartEvery: time.Hour * 24,
+	}
+)
+
+// Equals returns true if b equals p, otherwise false. If both sides are nil,
+// Equals returns true. If only one side is nil, Equals returns false.
+func (p *ProbeUDPLifetimeConfig) Equals(b *ProbeUDPLifetimeConfig) bool {
+	if p == b {
+		return true
+	}
+	if (p == nil && b != nil) || (b == nil && p != nil) {
+		return false
+	}
+	if !slices.Equal(p.Cliffs, b.Cliffs) {
+		return false
+	}
+	if p.CycleCanStartEvery != b.CycleCanStartEvery {
+		return false
+	}
+	return true
+}
+
+// Valid returns true if p is valid, otherwise false. p must be non-nil.
+func (p *ProbeUDPLifetimeConfig) Valid() bool {
+	if len(p.Cliffs) < 1 {
+		// We need at least one cliff, otherwise there is nothing to probe.
+		return false
+	}
+	if p.CycleCanStartEvery < 1 {
+		// Probing must be constrained by a positive CycleCanStartEvery.
+		return false
+	}
+	for i, c := range p.Cliffs {
+		if c <= max(udpLifetimeProbeCliffSlack*2, heartbeatInterval) {
+			// A timeout cliff less than or equal to twice
+			// udpLifetimeProbeCliffSlack is invalid due to being effectively
+			// zero when the cliff slack is subtracted from the cliff value at
+			// scheduling time.
+			//
+			// A timeout cliff less or equal to the heartbeatInterval is also
+			// invalid, as we may attempt to schedule on the tail end of the
+			// last heartbeat tied to an active session.
+			//
+			// These values are constants, but max()'d in case they change in
+			// the future.
+			return false
+		}
+		if i == 0 {
+			continue
+		}
+		if c <= p.Cliffs[i-1] {
+			// Cliffs must be in ascending order.
+			return false
+		}
+	}
+	return true
+}
+
+// setProbeUDPLifetimeOn enables or disables probing of UDP path lifetime based
+// on v. In the case of enablement defaultProbeUDPLifetimeConfig is used as the
+// desired configuration.
+func (de *endpoint) setProbeUDPLifetimeOn(v bool) {
+	de.mu.Lock()
+	if v {
+		de.setProbeUDPLifetimeConfigLocked(defaultProbeUDPLifetimeConfig)
+	} else {
+		de.setProbeUDPLifetimeConfigLocked(nil)
+	}
+	de.mu.Unlock()
+}
+
+// setProbeUDPLifetimeConfigLocked sets the desired configuration for probing
+// UDP path lifetime. Ownership of desired is passed to endpoint, it must not be
+// mutated once this call is made. A nil value disables the feature. If desired
+// is non-nil but desired.Valid() returns false this is a no-op.
+func (de *endpoint) setProbeUDPLifetimeConfigLocked(desired *ProbeUDPLifetimeConfig) {
+	if de.isWireguardOnly {
+		return
+	}
+	if desired == nil {
+		if de.probeUDPLifetime == nil {
+			// noop, not currently configured or desired
+			return
+		}
+		de.probeUDPLifetime.resetCycleEndpointLocked()
+		de.probeUDPLifetime = nil
+		return
+	}
+	if !desired.Valid() {
+		return
+	}
+	if de.probeUDPLifetime != nil {
+		if de.probeUDPLifetime.config.Equals(desired) {
+			// noop, current config equals desired
+			return
+		}
+		de.probeUDPLifetime.resetCycleEndpointLocked()
+	} else {
+		de.probeUDPLifetime = &probeUDPLifetime{}
+	}
+	p := de.probeUDPLifetime
+	p.config = *desired
+	p.resetCycleEndpointLocked()
+}
+
 // endpointDisco is the current disco key and short string for an endpoint. This
 // structure is immutable.
 type endpointDisco struct {
@@ -219,7 +451,7 @@ func (de *endpoint) deleteEndpointLocked(why string, ep netip.AddrPort) {
 			What: "deleteEndpointLocked-bestAddr-" + why,
 			From: de.bestAddr,
 		})
-		de.bestAddr = addrQuality{}
+		de.setBestAddrLocked(addrQuality{})
 	}
 }
 
@@ -236,9 +468,7 @@ func (de *endpoint) initFakeUDPAddr() {
 
 // noteRecvActivity records receive activity on de, and invokes
 // Conn.noteRecvActivity no more than once every 10s.
-func (de *endpoint) noteRecvActivity(ipp netip.AddrPort) {
-	now := mono.Now()
-
+func (de *endpoint) noteRecvActivity(ipp netip.AddrPort, now mono.Time) {
 	if de.isWireguardOnly {
 		de.mu.Lock()
 		de.bestAddr.AddrPort = ipp
@@ -257,9 +487,9 @@ func (de *endpoint) noteRecvActivity(ipp netip.AddrPort) {
 		de.mu.Unlock()
 	}
 
-	elapsed := now.Sub(de.lastRecv.LoadAtomic())
+	elapsed := now.Sub(de.lastRecvWG.LoadAtomic())
 	if elapsed > 10*time.Second {
-		de.lastRecv.StoreAtomic(now)
+		de.lastRecvWG.StoreAtomic(now)
 
 		if de.c.noteRecvActivity == nil {
 			return
@@ -410,12 +640,140 @@ func (de *endpoint) addrForPingSizeLocked(now mono.Time, size int) (udpAddr, der
 	return netip.AddrPort{}, de.derpAddr
 }
 
+// maybeProbeUDPLifetimeLocked returns an afterInactivityFor duration and true
+// if de is a candidate for UDP path lifetime probing in the future, otherwise
+// false.
+func (de *endpoint) maybeProbeUDPLifetimeLocked() (afterInactivityFor time.Duration, maybe bool) {
+	p := de.probeUDPLifetime
+	if p == nil {
+		return afterInactivityFor, false
+	}
+	if !de.bestAddr.IsValid() {
+		return afterInactivityFor, false
+	}
+	epDisco := de.disco.Load()
+	if epDisco == nil {
+		// peer does not support disco
+		return afterInactivityFor, false
+	}
+	// We compare disco keys, which may have a shorter lifetime than node keys
+	// since disco keys reset on startup. This has the desired side effect of
+	// shuffling probing probability where the local node ends up with a large
+	// key value lexicographically relative to the other nodes it tends to
+	// communicate with. If de's disco key changes, the cycle will reset.
+	if de.c.discoPublic.Compare(epDisco.key) >= 0 {
+		// lower disco pub key node probes higher
+		return afterInactivityFor, false
+	}
+	if !p.cycleActive && time.Since(p.cycleStartedAt) < p.config.CycleCanStartEvery {
+		// This is conservative as it doesn't account for afterInactivityFor use
+		// by the caller, potentially delaying the start of the next cycle. We
+		// assume the cycle could start immediately following
+		// maybeProbeUDPLifetimeLocked(), regardless of the value of
+		// afterInactivityFor relative to latest packets in/out time.
+		return afterInactivityFor, false
+	}
+	afterInactivityFor = p.currentCliffDurationEndpointLocked() - udpLifetimeProbeCliffSlack
+	if afterInactivityFor < 0 {
+		// shouldn't happen
+		return afterInactivityFor, false
+	}
+	return afterInactivityFor, true
+}
+
+// heartbeatForLifetimeVia represents the scheduling source of
+// endpoint.heartbeatForLifetime().
+type heartbeatForLifetimeVia string
+
+const (
+	heartbeatForLifetimeViaSessionInactive heartbeatForLifetimeVia = "session-inactive"
+	heartbeatForLifetimeViaPongRx          heartbeatForLifetimeVia = "pong-rx"
+	heartbeatForLifetimeViaSelf            heartbeatForLifetimeVia = "self"
+)
+
+// scheduleHeartbeatForLifetimeLocked schedules de.heartbeatForLifetime to fire
+// in the future (after). The caller must describe themselves in the via arg.
+func (de *endpoint) scheduleHeartbeatForLifetimeLocked(after time.Duration, via heartbeatForLifetimeVia) {
+	p := de.probeUDPLifetime
+	if p == nil {
+		return
+	}
+	de.c.dlogf("[v1] magicsock: disco: scheduling UDP lifetime probe for cliff=%v via=%v to %v (%v)",
+		p.currentCliffDurationEndpointLocked(), via, de.publicKey.ShortString(), de.discoShort())
+	p.bestAddr = de.bestAddr.AddrPort
+	p.timer = time.AfterFunc(after, de.heartbeatForLifetime)
+	if via == heartbeatForLifetimeViaSelf {
+		metricUDPLifetimeCliffsRescheduled.Add(1)
+	} else {
+		metricUDPLifetimeCliffsScheduled.Add(1)
+	}
+}
+
+// heartbeatForLifetime sends a disco ping recorded locally with a purpose of
+// pingHeartbeatForUDPLifetime to de if de.bestAddr has remained stable, and it
+// has been inactive for a duration that is within the error bounds for current
+// lifetime probing cliff. Alternatively it may reschedule itself into the
+// future, which is one of three scheduling sources. The other scheduling
+// sources are de.heartbeat() and de.probeUDPLifetimeCliffDoneLocked().
+func (de *endpoint) heartbeatForLifetime() {
+	de.mu.Lock()
+	defer de.mu.Unlock()
+	p := de.probeUDPLifetime
+	if p == nil || p.timer == nil {
+		// We raced with a code path trying to p.timer.Stop() us. Give up early
+		// in the interest of simplicity. If p.timer.Stop() happened in
+		// de.heartbeat() presumably because of recent packets in/out we *could*
+		// still probe here, and it would be meaningful, but the time logic
+		// below would reschedule as-is.
+		return
+	}
+	p.timer = nil
+	if !p.bestAddr.IsValid() || de.bestAddr.AddrPort != p.bestAddr {
+		// best path changed
+		p.resetCycleEndpointLocked()
+		return
+	}
+	afterInactivityFor, ok := de.maybeProbeUDPLifetimeLocked()
+	if !ok {
+		p.resetCycleEndpointLocked()
+		return
+	}
+	inactiveFor := mono.Now().Sub(max(de.lastRecvUDPAny.LoadAtomic(), de.lastSendAny))
+	delta := afterInactivityFor - inactiveFor
+	if delta.Abs() > udpLifetimeProbeSchedulingTolerance {
+		if delta < 0 {
+			// We missed our opportunity. We can resume this cliff at the tail
+			// end of another session.
+			metricUDPLifetimeCliffsMissed.Add(1)
+			return
+		} else {
+			// We need to wait longer before sending a ping. This can happen for
+			// a number of reasons, which are described in more detail in
+			// de.heartbeat().
+			de.scheduleHeartbeatForLifetimeLocked(delta, heartbeatForLifetimeViaSelf)
+			return
+		}
+	}
+	if p.currentCliff == 0 {
+		p.cycleStartedAt = time.Now()
+		p.cycleActive = true
+	}
+	de.c.dlogf("[v1] magicsock: disco: sending disco ping for UDP lifetime probe cliff=%v to %v (%v)",
+		p.currentCliffDurationEndpointLocked(), de.publicKey.ShortString(), de.discoShort())
+	de.startDiscoPingLocked(de.bestAddr.AddrPort, mono.Now(), pingHeartbeatForUDPLifetime, 0, nil)
+}
+
 // heartbeat is called every heartbeatInterval to keep the best UDP path alive,
-// or kick off discovery of other paths.
+// kick off discovery of other paths, or schedule the probing of UDP path
+// lifetime on the tail end of an active session.
 func (de *endpoint) heartbeat() {
 	de.mu.Lock()
 	defer de.mu.Unlock()
 
+	if de.probeUDPLifetime != nil && de.probeUDPLifetime.timer != nil {
+		de.probeUDPLifetime.timer.Stop()
+		de.probeUDPLifetime.timer = nil
+	}
 	de.heartBeatTimer = nil
 
 	if de.heartbeatDisabled {
@@ -423,18 +781,42 @@ func (de *endpoint) heartbeat() {
 		return
 	}
 
-	if de.lastSend.IsZero() {
+	if de.lastSendExt.IsZero() {
 		// Shouldn't happen.
 		return
 	}
 
-	if mono.Since(de.lastSend) > sessionActiveTimeout {
+	now := mono.Now()
+	if now.Sub(de.lastSendExt) > sessionActiveTimeout {
 		// Session's idle. Stop heartbeating.
 		de.c.dlogf("[v1] magicsock: disco: ending heartbeats for idle session to %v (%v)", de.publicKey.ShortString(), de.discoShort())
+		if afterInactivityFor, ok := de.maybeProbeUDPLifetimeLocked(); ok {
+			// This is the best place to best effort schedule a probe of UDP
+			// path lifetime in the future as it loosely translates to "UDP path
+			// is inactive".
+			//
+			// Note: wireguard-go schedules a WireGuard keepalive packet (by
+			// default, not tied to persistent keepalive feature) 10 seconds in
+			// the future after receiving an authenticated data packet. It's
+			// typically only sent by one side based on how the WireGuard state
+			// machine controls the timer. So, if we are on the receiving end of
+			// that keepalive, de.lastSendExt won't move, assuming there is no
+			// other user-generated traffic. This is one reason why we perform
+			// a more granular check of the last packets in/out time, below, as
+			// a WireGuard keepalive may have fallen somewhere within the
+			// sessionActiveTimeout window. heartbeatForLifetime will also
+			// perform a similar check, and reschedule as necessary.
+			inactiveFor := now.Sub(max(de.lastSendAny, de.lastRecvUDPAny.LoadAtomic()))
+			after := afterInactivityFor - inactiveFor
+			if after < 0 {
+				// shouldn't happen
+				return
+			}
+			de.scheduleHeartbeatForLifetimeLocked(after, heartbeatForLifetimeViaSessionInactive)
+		}
 		return
 	}
 
-	now := mono.Now()
 	udpAddr, _, _ := de.addrForSendLocked(now)
 	if udpAddr.IsValid() {
 		// We have a preferred path. Ping that every 2 seconds.
@@ -478,8 +860,8 @@ func (de *endpoint) wantFullPingLocked(now mono.Time) bool {
 	return false
 }
 
-func (de *endpoint) noteActiveLocked() {
-	de.lastSend = mono.Now()
+func (de *endpoint) noteTxActivityExtTriggerLocked(now mono.Time) {
+	de.lastSendExt = now
 	if de.heartBeatTimer == nil && !de.heartbeatDisabled {
 		de.heartBeatTimer = time.AfterFunc(heartbeatInterval, de.heartbeat)
 	}
@@ -536,7 +918,6 @@ func (de *endpoint) discoPing(res *ipnstate.PingResult, size int, cb func(*ipnst
 			de.startDiscoPingLocked(ep, now, pingCLI, size, resCB)
 		}
 	}
-	de.noteActiveLocked()
 }
 
 var (
@@ -562,7 +943,8 @@ func (de *endpoint) send(buffs [][]byte) error {
 	} else if !udpAddr.IsValid() || now.After(de.trustBestAddrUntil) {
 		de.sendDiscoPingsLocked(now, true)
 	}
-	de.noteActiveLocked()
+	de.noteTxActivityExtTriggerLocked(now)
+	de.lastSendAny = now
 	de.mu.Unlock()
 
 	if !udpAddr.IsValid() && !derpAddr.IsValid() {
@@ -606,6 +988,42 @@ func (de *endpoint) send(buffs [][]byte) error {
 	return err
 }
 
+// probeUDPLifetimeCliffDoneLocked is called when a disco
+// pingHeartbeatForUDPLifetime is being cleaned up. result contains the reason
+// for the cleanup, txid contains the ping's txid.
+// probeUDPLifetimeCliffDoneLocked may schedule another
+// pingHeartbeatForUDPLifetime in the future if there is another cliff remaining
+// for the current probing cycle.
+func (de *endpoint) probeUDPLifetimeCliffDoneLocked(result discoPingResult, txid stun.TxID) {
+	p := de.probeUDPLifetime
+	if p == nil || !p.cycleActive || de.probeUDPLifetime.timer != nil || txid != p.lastTxID {
+		// Probing may have been disabled while heartbeats were in flight. This
+		// can also be a duplicate or late arriving result.
+		return
+	}
+	metricUDPLifetimeCliffsCompleted.Add(1)
+	if result != discoPongReceived || p.currentCliff >= len(p.config.Cliffs)-1 {
+		maxCliffIndex := p.currentCliff
+		if result != discoPongReceived {
+			maxCliffIndex = p.currentCliff - 1
+		}
+		var maxCliffDuration time.Duration
+		if maxCliffIndex >= 0 {
+			maxCliffDuration = p.config.Cliffs[maxCliffIndex]
+		}
+		p.cycleCompleteMaxCliffEndpointLocked(maxCliffIndex)
+		de.c.dlogf("[v1] magicsock: disco: UDP lifetime probe cycle completed max cliff=%v for %v (%v)",
+			maxCliffDuration, de.publicKey.ShortString(), de.discoShort())
+		metricUDPLifetimeCyclesCompleted.Add(1)
+		p.resetCycleEndpointLocked()
+	} else {
+		p.currentCliff++
+		if after, ok := de.maybeProbeUDPLifetimeLocked(); ok {
+			de.scheduleHeartbeatForLifetimeLocked(after, heartbeatForLifetimeViaPongRx)
+		}
+	}
+}
+
 func (de *endpoint) discoPingTimeout(txid stun.TxID) {
 	de.mu.Lock()
 	defer de.mu.Unlock()
@@ -616,23 +1034,36 @@ func (de *endpoint) discoPingTimeout(txid stun.TxID) {
 	if debugDisco() || !de.bestAddr.IsValid() || mono.Now().After(de.trustBestAddrUntil) {
 		de.c.dlogf("[v1] magicsock: disco: timeout waiting for pong %x from %v (%v, %v)", txid[:6], sp.to, de.publicKey.ShortString(), de.discoShort())
 	}
-	de.removeSentDiscoPingLocked(txid, sp)
+	de.removeSentDiscoPingLocked(txid, sp, discoPingTimedOut)
 }
 
-// forgetDiscoPing is called by a timer when a ping either fails to send or
-// has taken too long to get a pong reply.
+// forgetDiscoPing is called when a ping fails to send.
 func (de *endpoint) forgetDiscoPing(txid stun.TxID) {
 	de.mu.Lock()
 	defer de.mu.Unlock()
 	if sp, ok := de.sentPing[txid]; ok {
-		de.removeSentDiscoPingLocked(txid, sp)
+		de.removeSentDiscoPingLocked(txid, sp, discoPingFailed)
 	}
 }
 
-func (de *endpoint) removeSentDiscoPingLocked(txid stun.TxID, sp sentPing) {
+// discoPingResult represents the result of an attempted disco ping send
+// operation.
+type discoPingResult int
+
+const (
+	discoPingResultUnknown discoPingResult = iota
+	discoPingFailed
+	discoPingTimedOut
+	discoPongReceived
+)
+
+func (de *endpoint) removeSentDiscoPingLocked(txid stun.TxID, sp sentPing, result discoPingResult) {
 	// Stop the timer for the case where sendPing failed to write to UDP.
 	// In the case of a timer already having fired, this is a no-op:
 	sp.timer.Stop()
+	if sp.purpose == pingHeartbeatForUDPLifetime {
+		de.probeUDPLifetimeCliffDoneLocked(result, txid)
+	}
 	delete(de.sentPing, txid)
 }
 
@@ -685,6 +1116,11 @@ const (
 	// pingCLI means that the user is running "tailscale ping"
 	// from the CLI. These types of pings can go over DERP.
 	pingCLI
+
+	// pingHeartbeatForUDPLifetime means that the purpose of a ping was to
+	// discover whether the UDP path was still active through any and all
+	// stateful middleboxes involved.
+	pingHeartbeatForUDPLifetime
 )
 
 // startDiscoPingLocked sends a disco ping to ep in a separate goroutine. resCB,
@@ -731,6 +1167,10 @@ func (de *endpoint) startDiscoPingLocked(ep netip.AddrPort, now mono.Time, purpo
 	if purpose == pingHeartbeat {
 		logLevel = discoVerboseLog
 	}
+	if purpose == pingCLI {
+		de.noteTxActivityExtTriggerLocked(now)
+	}
+	de.lastSendAny = now
 	for _, s := range sizes {
 		txid := stun.NewTxID()
 		de.sentPing[txid] = sentPing{
@@ -741,6 +1181,9 @@ func (de *endpoint) startDiscoPingLocked(ep netip.AddrPort, now mono.Time, purpo
 			resCB:   resCB,
 			size:    s,
 		}
+		if purpose == pingHeartbeatForUDPLifetime && de.probeUDPLifetime != nil {
+			de.probeUDPLifetime.lastTxID = txid
+		}
 		go de.sendDiscoPing(ep, epDisco.key, txid, s, logLevel)
 	}
 
@@ -864,7 +1307,7 @@ func (de *endpoint) setLastPing(ipp netip.AddrPort, now mono.Time) {
 
 // updateFromNode updates the endpoint based on a tailcfg.Node from a NetMap
 // update.
-func (de *endpoint) updateFromNode(n tailcfg.NodeView, heartbeatDisabled bool) {
+func (de *endpoint) updateFromNode(n tailcfg.NodeView, heartbeatDisabled bool, probeUDPLifetimeEnabled bool) {
 	if !n.Valid() {
 		panic("nil node when updating endpoint")
 	}
@@ -872,6 +1315,11 @@ func (de *endpoint) updateFromNode(n tailcfg.NodeView, heartbeatDisabled bool) {
 	defer de.mu.Unlock()
 
 	de.heartbeatDisabled = heartbeatDisabled
+	if probeUDPLifetimeEnabled {
+		de.setProbeUDPLifetimeConfigLocked(defaultProbeUDPLifetimeConfig)
+	} else {
+		de.setProbeUDPLifetimeConfigLocked(nil)
+	}
 	de.expired = n.Expired()
 
 	epDisco := de.disco.Load()
@@ -1009,7 +1457,7 @@ func (de *endpoint) addCandidateEndpoint(ep netip.AddrPort, forRxPingTxID stun.T
 //
 // de.mu must be held.
 func (de *endpoint) clearBestAddrLocked() {
-	de.bestAddr = addrQuality{}
+	de.setBestAddrLocked(addrQuality{})
 	de.bestAddrAt = 0
 	de.trustBestAddrUntil = 0
 }
@@ -1093,7 +1541,7 @@ func (de *endpoint) handlePongConnLocked(m *disco.Pong, di *discoInfo, src netip
 		return false
 	}
 	knownTxID = true // for naked returns below
-	de.removeSentDiscoPingLocked(m.TxID, sp)
+	de.removeSentDiscoPingLocked(m.TxID, sp, discoPongReceived)
 
 	pktLen := int(pingSizeToPktLen(sp.size, sp.to.Addr().Is6()))
 	if sp.size != 0 {
@@ -1124,7 +1572,7 @@ func (de *endpoint) handlePongConnLocked(m *disco.Pong, di *discoInfo, src netip
 		})
 	}
 
-	if sp.purpose != pingHeartbeat {
+	if sp.purpose != pingHeartbeat && sp.purpose != pingHeartbeatForUDPLifetime {
 		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)
@@ -1152,7 +1600,7 @@ func (de *endpoint) handlePongConnLocked(m *disco.Pong, di *discoInfo, src netip
 				From: de.bestAddr,
 				To:   thisPong,
 			})
-			de.bestAddr = thisPong
+			de.setBestAddrLocked(thisPong)
 		}
 		if de.bestAddr.AddrPort == thisPong.AddrPort {
 			de.debugUpdates.Add(EndpointChange{
@@ -1327,13 +1775,13 @@ func (de *endpoint) populatePeerStatus(ps *ipnstate.PeerStatus) {
 
 	ps.Relay = de.c.derpRegionCodeOfIDLocked(int(de.derpAddr.Port()))
 
-	if de.lastSend.IsZero() {
+	if de.lastSendExt.IsZero() {
 		return
 	}
 
 	now := mono.Now()
-	ps.LastWrite = de.lastSend.WallTime()
-	ps.Active = now.Sub(de.lastSend) < sessionActiveTimeout
+	ps.LastWrite = de.lastSendExt.WallTime()
+	ps.Active = now.Sub(de.lastSendExt) < sessionActiveTimeout
 
 	if udpAddr, derpAddr, _ := de.addrForSendLocked(now); udpAddr.IsValid() && !derpAddr.IsValid() {
 		ps.CurAddr = udpAddr.String()
@@ -1372,7 +1820,7 @@ func (de *endpoint) stopAndReset() {
 // DERP-only endpoint. It does not stop the endpoint's heartbeat
 // timer, if one is running.
 func (de *endpoint) resetLocked() {
-	de.lastSend = 0
+	de.lastSendExt = 0
 	de.lastFullPing = 0
 	de.clearBestAddrLocked()
 	for _, es := range de.endpointState {
@@ -1380,9 +1828,10 @@ func (de *endpoint) resetLocked() {
 	}
 	if !de.isWireguardOnly {
 		for txid, sp := range de.sentPing {
-			de.removeSentDiscoPingLocked(txid, sp)
+			de.removeSentDiscoPingLocked(txid, sp, discoPingResultUnknown)
 		}
 	}
+	de.probeUDPLifetime.resetCycleEndpointLocked()
 }
 
 func (de *endpoint) numStopAndReset() int64 {

wgengine/magicsock/endpoint_test.go

@@ -0,0 +1,326 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package magicsock
+
+import (
+	"net/netip"
+	"testing"
+	"time"
+
+	"github.com/dsnet/try"
+	"tailscale.com/types/key"
+)
+
+func TestProbeUDPLifetimeConfig_Equals(t *testing.T) {
+	tests := []struct {
+		name string
+		a    *ProbeUDPLifetimeConfig
+		b    *ProbeUDPLifetimeConfig
+		want bool
+	}{
+		{
+			"both sides nil",
+			nil,
+			nil,
+			true,
+		},
+		{
+			"equal pointers",
+			defaultProbeUDPLifetimeConfig,
+			defaultProbeUDPLifetimeConfig,
+			true,
+		},
+		{
+			"a nil",
+			nil,
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{time.Second},
+				CycleCanStartEvery: time.Hour,
+			},
+			false,
+		},
+		{
+			"b nil",
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{time.Second},
+				CycleCanStartEvery: time.Hour,
+			},
+			nil,
+			false,
+		},
+		{
+			"Cliffs unequal",
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{time.Second},
+				CycleCanStartEvery: time.Hour,
+			},
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{time.Second * 2},
+				CycleCanStartEvery: time.Hour,
+			},
+			false,
+		},
+		{
+			"CycleCanStartEvery unequal",
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{time.Second},
+				CycleCanStartEvery: time.Hour,
+			},
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{time.Second},
+				CycleCanStartEvery: time.Hour * 2,
+			},
+			false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.a.Equals(tt.b); got != tt.want {
+				t.Errorf("Equals() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestProbeUDPLifetimeConfig_Valid(t *testing.T) {
+	tests := []struct {
+		name string
+		p    *ProbeUDPLifetimeConfig
+		want bool
+	}{
+		{
+			"default config valid",
+			defaultProbeUDPLifetimeConfig,
+			true,
+		},
+		{
+			"no cliffs",
+			&ProbeUDPLifetimeConfig{
+				CycleCanStartEvery: time.Hour,
+			},
+			false,
+		},
+		{
+			"zero CycleCanStartEvery",
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{time.Second * 10},
+				CycleCanStartEvery: 0,
+			},
+			false,
+		},
+		{
+			"cliff too small",
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{min(udpLifetimeProbeCliffSlack*2, heartbeatInterval)},
+				CycleCanStartEvery: time.Hour,
+			},
+			false,
+		},
+		{
+			"duplicate Cliffs values",
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{time.Second * 2, time.Second * 2},
+				CycleCanStartEvery: time.Hour,
+			},
+			false,
+		},
+		{
+			"Cliffs not ascending",
+			&ProbeUDPLifetimeConfig{
+				Cliffs:             []time.Duration{time.Second * 2, time.Second * 1},
+				CycleCanStartEvery: time.Hour,
+			},
+			false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.p.Valid(); got != tt.want {
+				t.Errorf("Valid() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_endpoint_maybeProbeUDPLifetimeLocked(t *testing.T) {
+	var lower, higher key.DiscoPublic
+	a := key.NewDisco().Public()
+	b := key.NewDisco().Public()
+	if a.String() < b.String() {
+		lower = a
+		higher = b
+	} else {
+		lower = b
+		higher = a
+	}
+	addr := addrQuality{AddrPort: try.E1[netip.AddrPort](netip.ParseAddrPort("1.1.1.1:1"))}
+	newProbeUDPLifetime := func() *probeUDPLifetime {
+		return &probeUDPLifetime{
+			config: *defaultProbeUDPLifetimeConfig,
+		}
+	}
+
+	tests := []struct {
+		name                     string
+		localDisco               key.DiscoPublic
+		remoteDisco              *key.DiscoPublic
+		probeUDPLifetimeFn       func() *probeUDPLifetime
+		bestAddr                 addrQuality
+		wantAfterInactivityForFn func(*probeUDPLifetime) time.Duration
+		wantMaybe                bool
+	}{
+		{
+			"nil probeUDPLifetime",
+			higher,
+			&lower,
+			func() *probeUDPLifetime {
+				return nil
+			},
+			addr,
+			func(lifetime *probeUDPLifetime) time.Duration {
+				return 0
+			},
+			false,
+		},
+		{
+			"local higher disco key",
+			higher,
+			&lower,
+			newProbeUDPLifetime,
+			addr,
+			func(lifetime *probeUDPLifetime) time.Duration {
+				return 0
+			},
+			false,
+		},
+		{
+			"remote no disco key",
+			higher,
+			nil,
+			newProbeUDPLifetime,
+			addr,
+			func(lifetime *probeUDPLifetime) time.Duration {
+				return 0
+			},
+			false,
+		},
+		{
+			"invalid bestAddr",
+			lower,
+			&higher,
+			newProbeUDPLifetime,
+			addrQuality{},
+			func(lifetime *probeUDPLifetime) time.Duration {
+				return 0
+			},
+			false,
+		},
+		{
+			"cycle started too recently",
+			lower,
+			&higher,
+			func() *probeUDPLifetime {
+				l := newProbeUDPLifetime()
+				l.cycleActive = false
+				l.cycleStartedAt = time.Now()
+				return l
+			},
+			addr,
+			func(lifetime *probeUDPLifetime) time.Duration {
+				return 0
+			},
+			false,
+		},
+		{
+			"maybe cliff 0 cycle not active",
+			lower,
+			&higher,
+			func() *probeUDPLifetime {
+				l := newProbeUDPLifetime()
+				l.cycleActive = false
+				l.cycleStartedAt = time.Now().Add(-l.config.CycleCanStartEvery).Add(-time.Second)
+				return l
+			},
+			addr,
+			func(lifetime *probeUDPLifetime) time.Duration {
+				return lifetime.config.Cliffs[0] - udpLifetimeProbeCliffSlack
+			},
+			true,
+		},
+		{
+			"maybe cliff 0",
+			lower,
+			&higher,
+			func() *probeUDPLifetime {
+				l := newProbeUDPLifetime()
+				l.cycleActive = true
+				l.currentCliff = 0
+				return l
+			},
+			addr,
+			func(lifetime *probeUDPLifetime) time.Duration {
+				return lifetime.config.Cliffs[0] - udpLifetimeProbeCliffSlack
+			},
+			true,
+		},
+		{
+			"maybe cliff 1",
+			lower,
+			&higher,
+			func() *probeUDPLifetime {
+				l := newProbeUDPLifetime()
+				l.cycleActive = true
+				l.currentCliff = 1
+				return l
+			},
+			addr,
+			func(lifetime *probeUDPLifetime) time.Duration {
+				return lifetime.config.Cliffs[1] - udpLifetimeProbeCliffSlack
+			},
+			true,
+		},
+		{
+			"maybe cliff 2",
+			lower,
+			&higher,
+			func() *probeUDPLifetime {
+				l := newProbeUDPLifetime()
+				l.cycleActive = true
+				l.currentCliff = 2
+				return l
+			},
+			addr,
+			func(lifetime *probeUDPLifetime) time.Duration {
+				return lifetime.config.Cliffs[2] - udpLifetimeProbeCliffSlack
+			},
+			true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			de := &endpoint{
+				c: &Conn{
+					discoPublic: tt.localDisco,
+				},
+				bestAddr: tt.bestAddr,
+			}
+			if tt.remoteDisco != nil {
+				remote := &endpointDisco{
+					key: *tt.remoteDisco,
+				}
+				de.disco.Store(remote)
+			}
+			p := tt.probeUDPLifetimeFn()
+			de.probeUDPLifetime = p
+			gotAfterInactivityFor, gotMaybe := de.maybeProbeUDPLifetimeLocked()
+			wantAfterInactivityFor := tt.wantAfterInactivityForFn(p)
+			if gotAfterInactivityFor != wantAfterInactivityFor {
+				t.Errorf("maybeProbeUDPLifetimeLocked() gotAfterInactivityFor = %v, want %v", gotAfterInactivityFor, wantAfterInactivityFor)
+			}
+			if gotMaybe != tt.wantMaybe {
+				t.Errorf("maybeProbeUDPLifetimeLocked() gotMaybe = %v, want %v", gotMaybe, tt.wantMaybe)
+			}
+		})
+	}
+}

wgengine/magicsock/magicsock.go

@@ -141,6 +141,8 @@ type Conn struct {
 
 	silentDiscoOn atomic.Bool // whether silent disco is enabled
 
+	probeUDPLifetimeOn atomic.Bool // whether probing of UDP lifetime is enabled
+
 	// noV4Send is whether IPv4 UDP is known to be unable to transmit
 	// at all. This could happen if the socket is in an invalid state
 	// (as can happen on darwin after a network link status change).
@@ -749,7 +751,7 @@ func (c *Conn) LastRecvActivityOfNodeKey(nk key.NodePublic) string {
 	if !ok {
 		return "never"
 	}
-	saw := de.lastRecv.LoadAtomic()
+	saw := de.lastRecvWG.LoadAtomic()
 	if saw == 0 {
 		return "never"
 	}
@@ -1236,7 +1238,9 @@ func (c *Conn) receiveIP(b []byte, ipp netip.AddrPort, cache *ippEndpointCache)
 		cache.gen = de.numStopAndReset()
 		ep = de
 	}
-	ep.noteRecvActivity(ipp)
+	now := mono.Now()
+	ep.lastRecvUDPAny.StoreAtomic(now)
+	ep.noteRecvActivity(ipp, now)
 	if stats := c.stats.Load(); stats != nil {
 		stats.UpdateRxPhysical(ep.nodeAddr, ipp, len(b))
 	}
@@ -1383,6 +1387,15 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netip.AddrPort, derpNodeSrc ke
 		return
 	}
 
+	isDERP := src.Addr() == tailcfg.DerpMagicIPAddr
+	if !isDERP {
+		// Record receive time for UDP transport packets.
+		pi, ok := c.peerMap.byIPPort[src]
+		if ok {
+			pi.ep.lastRecvUDPAny.StoreAtomic(mono.Now())
+		}
+	}
+
 	// We're now reasonably sure we're expecting communication from
 	// this peer, do the heavy crypto lifting to see what they want.
 	//
@@ -1430,7 +1443,6 @@ func (c *Conn) handleDiscoMessage(msg []byte, src netip.AddrPort, derpNodeSrc ke
 		return
 	}
 
-	isDERP := src.Addr() == tailcfg.DerpMagicIPAddr
 	if isDERP {
 		metricRecvDiscoDERP.Add(1)
 	} else {
@@ -1817,11 +1829,13 @@ func debugRingBufferSize(numPeers int) int {
 // They might be set by envknob and/or controlknob.
 // The value is comparable.
 type debugFlags struct {
-	heartbeatDisabled bool
+	heartbeatDisabled  bool
+	probeUDPLifetimeOn bool
 }
 
 func (c *Conn) debugFlagsLocked() (f debugFlags) {
 	f.heartbeatDisabled = debugEnableSilentDisco() || c.silentDiscoOn.Load()
+	f.probeUDPLifetimeOn = c.probeUDPLifetimeOn.Load()
 	return
 }
 
@@ -1846,6 +1860,19 @@ func (c *Conn) SilentDisco() bool {
 	return flags.heartbeatDisabled
 }
 
+// SetProbeUDPLifetime toggles probing of UDP lifetime based on v.
+func (c *Conn) SetProbeUDPLifetime(v bool) {
+	old := c.probeUDPLifetimeOn.Swap(v)
+	if old == v {
+		return
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.peerMap.forEachEndpoint(func(ep *endpoint) {
+		ep.setProbeUDPLifetimeOn(v)
+	})
+}
+
 // SetNetworkMap is called when the control client gets a new network
 // map from the control server. It must always be non-nil.
 //
@@ -1876,7 +1903,8 @@ func (c *Conn) SetNetworkMap(nm *netmap.NetworkMap) {
 	if nodesEqual(priorPeers, curPeers) && c.lastFlags == flags {
 		// The rest of this function is all adjusting state for peers that have
 		// changed. But if the set of peers is equal and the debug flags (for
-		// silent disco) haven't changed, no need to do anything else.
+		// silent disco and probe UDP lifetime) haven't changed, there is no
+		// need to do anything else.
 		return
 	}
 
@@ -1927,7 +1955,7 @@ func (c *Conn) SetNetworkMap(nm *netmap.NetworkMap) {
 			if epDisco := ep.disco.Load(); epDisco != nil {
 				oldDiscoKey = epDisco.key
 			}
-			ep.updateFromNode(n, flags.heartbeatDisabled)
+			ep.updateFromNode(n, flags.heartbeatDisabled, flags.probeUDPLifetimeOn)
 			c.peerMap.upsertEndpoint(ep, oldDiscoKey) // maybe update discokey mappings in peerMap
 			continue
 		}
@@ -1980,7 +2008,7 @@ func (c *Conn) SetNetworkMap(nm *netmap.NetworkMap) {
 			c.logEndpointCreated(n)
 		}
 
-		ep.updateFromNode(n, flags.heartbeatDisabled)
+		ep.updateFromNode(n, flags.heartbeatDisabled, flags.probeUDPLifetimeOn)
 		c.peerMap.upsertEndpoint(ep, key.DiscoPublic{})
 	}
 
@@ -2947,8 +2975,34 @@ var (
 	// 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]
+
+	// metricUDPLifetime* metrics pertain to UDP lifetime probing, see type
+	// probeUDPLifetime. These metrics assume a static/default configuration for
+	// probing (defaultProbeUDPLifetimeConfig) until we disseminate
+	// ProbeUDPLifetimeConfig from control, and have lifetime management (GC old
+	// metrics) of clientmetrics or similar.
+	metricUDPLifetimeCliffsScheduled             = newUDPLifetimeCounter("magicsock_udp_lifetime_cliffs_scheduled")
+	metricUDPLifetimeCliffsCompleted             = newUDPLifetimeCounter("magicsock_udp_lifetime_cliffs_completed")
+	metricUDPLifetimeCliffsMissed                = newUDPLifetimeCounter("magicsock_udp_lifetime_cliffs_missed")
+	metricUDPLifetimeCliffsRescheduled           = newUDPLifetimeCounter("magicsock_udp_lifetime_cliffs_rescheduled")
+	metricUDPLifetimeCyclesCompleted             = newUDPLifetimeCounter("magicsock_udp_lifetime_cycles_completed")
+	metricUDPLifetimeCycleCompleteNoCliffReached = newUDPLifetimeCounter("magicsock_udp_lifetime_cycle_complete_no_cliff_reached")
+	metricUDPLifetimeCycleCompleteAt10sCliff     = newUDPLifetimeCounter("magicsock_udp_lifetime_cycle_complete_at_10s_cliff")
+	metricUDPLifetimeCycleCompleteAt30sCliff     = newUDPLifetimeCounter("magicsock_udp_lifetime_cycle_complete_at_30s_cliff")
+	metricUDPLifetimeCycleCompleteAt60sCliff     = newUDPLifetimeCounter("magicsock_udp_lifetime_cycle_complete_at_60s_cliff")
 )
 
+// newUDPLifetimeCounter returns a new *clientmetric.Metric with the provided
+// name combined with a suffix representing defaultProbeUDPLifetimeConfig.
+func newUDPLifetimeCounter(name string) *clientmetric.Metric {
+	var sb strings.Builder
+	for _, cliff := range defaultProbeUDPLifetimeConfig.Cliffs {
+		sb.WriteString(fmt.Sprintf("%ds", cliff/time.Second))
+	}
+	sb.WriteString(fmt.Sprintf("_%ds", defaultProbeUDPLifetimeConfig.CycleCanStartEvery/time.Second))
+	return clientmetric.NewCounter(fmt.Sprintf("%s_%s", name, sb.String()))
+}
+
 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) })

wgengine/magicsock/magicsock_test.go

@@ -1220,15 +1220,15 @@ func Test32bitAlignment(t *testing.T) {
 		},
 	}
 
-	if off := unsafe.Offsetof(de.lastRecv); off%8 != 0 {
-		t.Fatalf("endpoint.lastRecv is not 8-byte aligned")
+	if off := unsafe.Offsetof(de.lastRecvWG); off%8 != 0 {
+		t.Fatalf("endpoint.lastRecvWG is not 8-byte aligned")
 	}
 
-	de.noteRecvActivity(netip.AddrPort{}) // verify this doesn't panic on 32-bit
+	de.noteRecvActivity(netip.AddrPort{}, mono.Now()) // verify this doesn't panic on 32-bit
 	if called != 1 {
 		t.Fatal("expected call to noteRecvActivity")
 	}
-	de.noteRecvActivity(netip.AddrPort{})
+	de.noteRecvActivity(netip.AddrPort{}, mono.Now())
 	if called != 1 {
 		t.Error("expected no second call to noteRecvActivity")
 	}