Implement remaining multicast routing table operations.

In particular, this change adds support for AddInstalledRoute,
RemoveInstalledRoute, and GetLastUsedTimestamp.

Updates #7338.

PiperOrigin-RevId: 445205505

pkg/tcpip/network/internal/multicast/BUILD

@@ -9,7 +9,6 @@ go_library(
     ],
     visibility = ["//visibility:public"],
     deps = [
-        "//pkg/atomicbitops",
         "//pkg/tcpip",
         "//pkg/tcpip/stack",
     ],
@@ -21,7 +20,6 @@ go_test(
     srcs = ["route_table_test.go"],
     library = ":multicast",
     deps = [
-        "//pkg/atomicbitops",
         "//pkg/refs",
         "//pkg/refsvfs2",
         "//pkg/tcpip",
@@ -42,6 +40,7 @@ go_test(
         ":multicast",
         "//pkg/refs",
         "//pkg/refsvfs2",
+        "//pkg/tcpip",
         "//pkg/tcpip/buffer",
         "//pkg/tcpip/faketime",
         "//pkg/tcpip/stack",

pkg/tcpip/network/internal/multicast/example_test.go

@@ -18,9 +18,11 @@ import (
 	"fmt"
 	"os"
 	"testing"
+	"time"
 
 	"gvisor.dev/gvisor/pkg/refs"
 	"gvisor.dev/gvisor/pkg/refsvfs2"
+	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
 	"gvisor.dev/gvisor/pkg/tcpip/faketime"
 	"gvisor.dev/gvisor/pkg/tcpip/network/internal/multicast"
@@ -28,17 +30,27 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/testutil"
 )
 
+const (
+	defaultMinTTL             = 10
+	inputNICID    tcpip.NICID = 1
+	outgoingNICID tcpip.NICID = 2
+)
+
 // Example shows how to interact with a multicast RouteTable.
 func Example() {
 	address := testutil.MustParse4("192.168.1.1")
+	defaultOutgoingInterfaces := []multicast.OutgoingInterface{{ID: outgoingNICID, MinTTL: defaultMinTTL}}
 	routeKey := multicast.RouteKey{UnicastSource: address, MulticastDestination: address}
 
 	pkt := newPacketBuffer("hello")
 	defer pkt.DecRef()
 
+	clock := faketime.NewManualClock()
+	clock.Advance(10 * time.Second)
+
 	// Create a route table from a specified config.
 	table := multicast.RouteTable{}
-	config := multicast.DefaultConfig(faketime.NewManualClock())
+	config := multicast.DefaultConfig(clock)
 
 	if err := table.Init(config); err != nil {
 		panic(err)
@@ -72,12 +84,48 @@ func Example() {
 		deliverPktLocally(pkt)
 	}
 
+	// To transition a pending route to the installed state, call:
+	route := table.NewInstalledRoute(inputNICID, defaultOutgoingInterfaces)
+	pendingRoute, ok := table.AddInstalledRoute(routeKey, route)
+
+	if !ok {
+		return
+	}
+
+	// If there was a pending route, then the caller is responsible for
+	// flushing any pending packets.
+	for !pendingRoute.IsEmpty() {
+		pkt, err := pendingRoute.Dequeue()
+		if err != nil {
+			panic(fmt.Sprintf("pendingRoute.Dequeue() = (_, %s)", err))
+		}
+		forwardPkt(pkt, route)
+	}
+
+	// To obtain the last used time of the route, call:
+	timestamp, found := table.GetLastUsedTimestamp(routeKey)
+
+	if !found {
+		panic(fmt.Sprintf("table.GetLastUsedTimestamp(%#v) = (_, false)", routeKey))
+	}
+
+	fmt.Printf("Last used timestamp: %d", timestamp.Nanoseconds())
+
+	// Finally, to remove an installed route, call:
+	if removed := table.RemoveInstalledRoute(routeKey); !removed {
+		panic(fmt.Sprintf("table.RemoveInstalledRoute(%#v) = false", routeKey))
+	}
+
 	// Output:
 	// emitMissingRouteEvent
 	// deliverPktLocally
+	// forwardPkt
+	// Last used timestamp: 10000000000
 }
 
-func forwardPkt(*stack.PacketBuffer, *multicast.InstalledRoute) {}
+func forwardPkt(*stack.PacketBuffer, *multicast.InstalledRoute) {
+	fmt.Println("forwardPkt")
+}
 
 func emitMissingRouteEvent(multicast.RouteKey) {
 	fmt.Println("emitMissingRouteEvent")

pkg/tcpip/network/internal/multicast/route_table.go

@@ -19,9 +19,7 @@ import (
 	"errors"
 	"fmt"
 	"sync"
-	"time"
 
-	"gvisor.dev/gvisor/pkg/atomicbitops"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
@@ -55,7 +53,7 @@ type RouteTable struct {
 
 	pendingMu sync.RWMutex
 	// +checklocks:pendingMu
-	pendingRoutes map[RouteKey]pendingRoute
+	pendingRoutes map[RouteKey]PendingRoute
 
 	config Config
 }
@@ -69,7 +67,7 @@ var (
 	// Config, but is required.
 	ErrMissingClock = errors.New("clock must not be nil")
 
-	// ErrAlreadyInitialized indicate that RouteTable.Init was already invoked.
+	// ErrAlreadyInitialized indicates that RouteTable.Init was already invoked.
 	ErrAlreadyInitialized = errors.New("table is already initialized")
 )
 
@@ -86,8 +84,10 @@ type RouteKey struct {
 type InstalledRoute struct {
 	expectedInputInterface tcpip.NICID
 	outgoingInterfaces     []OutgoingInterface
-	// +checkatomic
-	lastUsedTimestamp atomicbitops.Int64
+
+	lastUsedTimestampMu sync.RWMutex
+	// +checklocks:lastUsedTimestampMu
+	lastUsedTimestamp tcpip.MonotonicTime
 }
 
 // ExpectedInputInterface returns the expected input interface for the route.
@@ -100,17 +100,27 @@ func (r *InstalledRoute) OutgoingInterfaces() []OutgoingInterface {
 	return r.outgoingInterfaces
 }
 
-// LastUsedTimestamp returns a Unix based timestamp in microseconds that
-// corresponds to the last time the route was used or updated.
-func (r *InstalledRoute) LastUsedTimestamp() int64 {
-	return r.lastUsedTimestamp.Load()
+// LastUsedTimestamp returns a monotonic timestamp that corresponds to the last
+// time the route was used or updated.
+func (r *InstalledRoute) LastUsedTimestamp() tcpip.MonotonicTime {
+	r.lastUsedTimestampMu.RLock()
+	defer r.lastUsedTimestampMu.RUnlock()
+
+	return r.lastUsedTimestamp
 }
 
 // SetLastUsedTimestamp sets the time that the route was last used.
 //
-// Callers should invoke this anytime the route is used to forward a packet.
-func (r *InstalledRoute) SetLastUsedTimestamp(time time.Time) {
-	r.lastUsedTimestamp.Store(time.UnixMicro())
+// The timestamp is only updated if it occurs after the currently set
+// timestamp. Callers should invoke this anytime the route is used to forward a
+// packet.
+func (r *InstalledRoute) SetLastUsedTimestamp(monotonicTime tcpip.MonotonicTime) {
+	r.lastUsedTimestampMu.Lock()
+	defer r.lastUsedTimestampMu.Unlock()
+
+	if monotonicTime.After(r.lastUsedTimestamp) {
+		r.lastUsedTimestamp = monotonicTime
+	}
 }
 
 // OutgoingInterface represents an interface that packets should be forwarded
@@ -124,23 +134,26 @@ type OutgoingInterface struct {
 	MinTTL uint8
 }
 
-// pendingRoute represents a route that is in the "pending" state.
+// PendingRoute represents a route that is in the "pending" state.
 //
 // A route is in the pending state if an installed route does not yet exist
 // for the entry. For such routes, packets are added to an expiring queue until
 // a route is installed.
-type pendingRoute struct {
+type PendingRoute struct {
 	packets []*stack.PacketBuffer
 }
 
-func newPendingRoute(maxSize uint8) pendingRoute {
-	return pendingRoute{packets: make([]*stack.PacketBuffer, 0, maxSize)}
+func newPendingRoute(maxSize uint8) PendingRoute {
+	return PendingRoute{packets: make([]*stack.PacketBuffer, 0, maxSize)}
 }
 
 // Dequeue removes the first element in the queue and returns it.
 //
 // If the queue is empty, then an error will be returned.
-func (p *pendingRoute) Dequeue() (*stack.PacketBuffer, error) {
+//
+// TODO(https://gvisor.dev/issue/7338): Remove this and instead just return the
+// list of packets from AddInstalledRoute.
+func (p *PendingRoute) Dequeue() (*stack.PacketBuffer, error) {
 	if len(p.packets) == 0 {
 		return nil, errors.New("dequeue called on queue empty")
 	}
@@ -152,7 +165,7 @@ func (p *pendingRoute) Dequeue() (*stack.PacketBuffer, error) {
 
 // IsEmpty returns true if the queue contains no more elements. Otherwise,
 // returns false.
-func (p *pendingRoute) IsEmpty() bool {
+func (p *PendingRoute) IsEmpty() bool {
 	return len(p.packets) == 0
 }
 
@@ -205,7 +218,7 @@ func (r *RouteTable) Init(config Config) error {
 
 	r.config = config
 	r.installedRoutes = make(map[RouteKey]*InstalledRoute)
-	r.pendingRoutes = make(map[RouteKey]pendingRoute)
+	r.pendingRoutes = make(map[RouteKey]PendingRoute)
 	return nil
 }
 
@@ -214,7 +227,7 @@ func (r *RouteTable) NewInstalledRoute(inputInterface tcpip.NICID, outgoingInter
 	return &InstalledRoute{
 		expectedInputInterface: inputInterface,
 		outgoingInterfaces:     outgoingInterfaces,
-		lastUsedTimestamp:      atomicbitops.FromInt64(r.config.Clock.Now().UnixMicro()),
+		lastUsedTimestamp:      r.config.Clock.NowMonotonic(),
 	}
 }
 
@@ -297,7 +310,7 @@ func (r *RouteTable) GetRouteOrInsertPending(key RouteKey, pkt *stack.PacketBuff
 }
 
 // +checklocks:r.pendingMu
-func (r *RouteTable) getOrCreatePendingRouteRLocked(key RouteKey) (pendingRoute, PendingRouteState) {
+func (r *RouteTable) getOrCreatePendingRouteRLocked(key RouteKey) (PendingRoute, PendingRouteState) {
 	if pendingRoute, ok := r.pendingRoutes[key]; ok {
 		return pendingRoute, PendingRouteStateAppended
 	}
@@ -305,3 +318,54 @@ func (r *RouteTable) getOrCreatePendingRouteRLocked(key RouteKey) (pendingRoute,
 	pendingRoute := newPendingRoute(r.config.MaxPendingQueueSize)
 	return pendingRoute, PendingRouteStateInstalled
 }
+
+// AddInstalledRoute adds the provided route to the table.
+//
+// Returns true if the route was previously in the pending state. Otherwise,
+// returns false.
+//
+// If the route was previously pending, then the caller is responsible for
+// flushing the returned pending route packet queue. Conversely, if the route
+// was not pending, then any existing installed route will be overwritten.
+func (r *RouteTable) AddInstalledRoute(key RouteKey, route *InstalledRoute) (PendingRoute, bool) {
+	r.installedMu.Lock()
+	defer r.installedMu.Unlock()
+	r.installedRoutes[key] = route
+
+	r.pendingMu.Lock()
+	defer r.pendingMu.Unlock()
+
+	pendingRoute, ok := r.pendingRoutes[key]
+	delete(r.pendingRoutes, key)
+	return pendingRoute, ok
+}
+
+// RemoveInstalledRoute deletes the installed route that matches the provided
+// key.
+//
+// Returns true if a route was removed. Otherwise returns false.
+func (r *RouteTable) RemoveInstalledRoute(key RouteKey) bool {
+	r.installedMu.Lock()
+	defer r.installedMu.Unlock()
+
+	if _, ok := r.installedRoutes[key]; ok {
+		delete(r.installedRoutes, key)
+		return true
+	}
+
+	return false
+}
+
+// GetLastUsedTimestamp returns a monotonic timestamp that represents the last
+// time the route that matches the provided key was used or updated.
+//
+// Returns true if a matching route was found. Otherwise returns false.
+func (r *RouteTable) GetLastUsedTimestamp(key RouteKey) (tcpip.MonotonicTime, bool) {
+	r.installedMu.RLock()
+	defer r.installedMu.RUnlock()
+
+	if route, ok := r.installedRoutes[key]; ok {
+		return route.LastUsedTimestamp(), true
+	}
+	return tcpip.MonotonicTime{}, false
+}

pkg/tcpip/network/internal/multicast/route_table_test.go

@@ -21,7 +21,6 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
-	"gvisor.dev/gvisor/pkg/atomicbitops"
 	"gvisor.dev/gvisor/pkg/refs"
 	"gvisor.dev/gvisor/pkg/refsvfs2"
 	"gvisor.dev/gvisor/pkg/tcpip"
@@ -35,6 +34,7 @@ const (
 	defaultMinTTL             = 10
 	inputNICID    tcpip.NICID = 1
 	outgoingNICID tcpip.NICID = 2
+	defaultNICID  tcpip.NICID = 3
 )
 
 var (
@@ -76,6 +76,18 @@ func defaultConfig(opts ...configOption) Config {
 	return *c
 }
 
+func installedRouteComparer(a *InstalledRoute, b *InstalledRoute) bool {
+	if !cmp.Equal(a.OutgoingInterfaces(), b.OutgoingInterfaces()) {
+		return false
+	}
+
+	if a.ExpectedInputInterface() != b.ExpectedInputInterface() {
+		return false
+	}
+
+	return a.LastUsedTimestamp() == b.LastUsedTimestamp()
+}
+
 func TestInit(t *testing.T) {
 	tests := []struct {
 		name        string
@@ -130,19 +142,9 @@ func TestNewInstalledRoute(t *testing.T) {
 	}
 
 	route := table.NewInstalledRoute(inputNICID, defaultOutgoingInterfaces)
-	expectedRoute := &InstalledRoute{expectedInputInterface: inputNICID, outgoingInterfaces: defaultOutgoingInterfaces, lastUsedTimestamp: atomicbitops.FromInt64(clock.Now().UnixMicro())}
+	expectedRoute := &InstalledRoute{expectedInputInterface: inputNICID, outgoingInterfaces: defaultOutgoingInterfaces, lastUsedTimestamp: clock.NowMonotonic()}
 
-	if diff := cmp.Diff(expectedRoute, route, cmp.Comparer(func(a *InstalledRoute, b *InstalledRoute) bool {
-		if !cmp.Equal(a.OutgoingInterfaces(), b.OutgoingInterfaces()) {
-			return false
-		}
-
-		if a.ExpectedInputInterface() != b.ExpectedInputInterface() {
-			return false
-		}
-
-		return a.LastUsedTimestamp() == b.LastUsedTimestamp()
-	})); diff != "" {
+	if diff := cmp.Diff(expectedRoute, route, cmp.Comparer(installedRouteComparer)); diff != "" {
 		t.Errorf("installed route mismatch (-want +got):\n%s", diff)
 	}
 }
@@ -158,7 +160,7 @@ func TestPendingRouteStates(t *testing.T) {
 	defer pkt.DecRef()
 	// Queue two pending packets for the same route. The PendingRouteState should
 	// transition from PendingRouteStateInstalled to PendingRouteStateAppended.
-	for _, wantPendingRouteState := range [...]PendingRouteState{PendingRouteStateInstalled, PendingRouteStateAppended} {
+	for _, wantPendingRouteState := range []PendingRouteState{PendingRouteStateInstalled, PendingRouteStateAppended} {
 		routeResult, err := table.GetRouteOrInsertPending(defaultRouteKey, pkt)
 
 		if err != nil {
@@ -178,6 +180,194 @@ func TestPendingRouteStates(t *testing.T) {
 	}
 }
 
+func TestAddInstalledRouteWithPending(t *testing.T) {
+	table := RouteTable{}
+	config := defaultConfig()
+	if err := table.Init(config); err != nil {
+		t.Fatalf("table.Init(%#v): %s", config, err)
+	}
+
+	wantPkt := newPacketBuffer("hello")
+	defer wantPkt.DecRef()
+	// Queue a pending packet. This packet should later be returned in a
+	// PendingRoute when table.AddInstalledRoute is invoked.
+	_, err := table.GetRouteOrInsertPending(defaultRouteKey, wantPkt)
+
+	if err != nil {
+		t.Fatalf("table.GetRouteOrInsertPending(%#v, %#v): %v", defaultRouteKey, wantPkt, err)
+	}
+
+	route := table.NewInstalledRoute(inputNICID, defaultOutgoingInterfaces)
+
+	pendingRoute, wasPending := table.AddInstalledRoute(defaultRouteKey, route)
+	if !wasPending {
+		t.Fatalf("got table.AddInstalledRoute(%#v, %#v) = (nil, false), want = (_, true)", defaultRouteKey, route)
+	}
+
+	// Verify that packets are properly dequeued from the PendingRoute.
+	pkt, err := pendingRoute.Dequeue()
+
+	if err != nil {
+		t.Fatalf("got pendingRoute.Dequeue() = (_, %v), want = (_, nil)", err)
+	}
+
+	if !cmp.Equal(wantPkt.Views(), pkt.Views()) {
+		t.Errorf("got pendingRoute.Dequeue() = (%v, nil), want = (%v, nil)", pkt.Views(), wantPkt.Views())
+	}
+
+	if !pendingRoute.IsEmpty() {
+		t.Errorf("got pendingRoute.IsEmpty() = false, want = true")
+	}
+
+	// Verify that the pending route is deleted (not returned on subsequent
+	// calls to AddInstalledRoute).
+	pendingRoute, wasPending = table.AddInstalledRoute(defaultRouteKey, route)
+	if wasPending {
+		t.Errorf("got table.AddInstalledRoute(%#v, %#v) = (%#v, true), want (_, false)", defaultRouteKey, route, pendingRoute)
+	}
+}
+
+func TestAddInstalledRouteWithNoPending(t *testing.T) {
+	table := RouteTable{}
+	config := defaultConfig()
+	if err := table.Init(config); err != nil {
+		t.Fatalf("table.Init(%#v): %s", config, err)
+	}
+
+	firstRoute := table.NewInstalledRoute(inputNICID, defaultOutgoingInterfaces)
+	secondRoute := table.NewInstalledRoute(defaultNICID, defaultOutgoingInterfaces)
+
+	pkt := newPacketBuffer("hello")
+	defer pkt.DecRef()
+	for _, route := range [...]*InstalledRoute{firstRoute, secondRoute} {
+		if pendingRoute, wasPending := table.AddInstalledRoute(defaultRouteKey, route); wasPending {
+			t.Errorf("got table.AddInstalledRoute(%#v, %#v) = (%#v, true), want = (_, false)", defaultRouteKey, route, pendingRoute)
+		}
+
+		// AddInstalledRoute is invoked for the same routeKey two times. Verify
+		// that the fetched InstalledRoute reflects the most recent invocation of
+		// AddInstalledRoute.
+		routeResult, err := table.GetRouteOrInsertPending(defaultRouteKey, pkt)
+
+		if err != nil {
+			t.Fatalf("table.GetRouteOrInsertPending(%#v, %#v): %v", defaultRouteKey, pkt, err)
+		}
+
+		if routeResult.PendingRouteState != PendingRouteStateNone {
+			t.Errorf("got routeResult.PendingRouteState = %s, want = PendingRouteStateNone", routeResult.PendingRouteState)
+		}
+
+		if diff := cmp.Diff(route, routeResult.InstalledRoute, cmp.Comparer(installedRouteComparer)); diff != "" {
+			t.Errorf("route.InstalledRoute mismatch (-want +got):\n%s", diff)
+		}
+	}
+}
+
+func TestRemoveInstalledRoute(t *testing.T) {
+	table := RouteTable{}
+	config := defaultConfig()
+	if err := table.Init(config); err != nil {
+		t.Fatalf("table.Init(%#v): %s", config, err)
+	}
+
+	route := table.NewInstalledRoute(inputNICID, defaultOutgoingInterfaces)
+
+	table.AddInstalledRoute(defaultRouteKey, route)
+
+	if removed := table.RemoveInstalledRoute(defaultRouteKey); !removed {
+		t.Errorf("got table.RemoveInstalledRoute(%#v) = false, want = true", defaultRouteKey)
+	}
+
+	pkt := newPacketBuffer("hello")
+	defer pkt.DecRef()
+
+	result, err := table.GetRouteOrInsertPending(defaultRouteKey, pkt)
+
+	if err != nil {
+		t.Fatalf("table.GetRouteOrInsertPending(%#v, %#v): %v", defaultRouteKey, pkt, err)
+	}
+
+	if result.InstalledRoute != nil {
+		t.Errorf("got result.InstalledRoute = %v, want = nil", result.InstalledRoute)
+	}
+}
+
+func TestRemoveInstalledRouteWithNoMatchingRoute(t *testing.T) {
+	table := RouteTable{}
+	config := defaultConfig()
+	if err := table.Init(config); err != nil {
+		t.Fatalf("table.Init(%#v): %s", config, err)
+	}
+
+	if removed := table.RemoveInstalledRoute(defaultRouteKey); removed {
+		t.Errorf("got table.RemoveInstalledRoute(%#v) = true, want = false", defaultRouteKey)
+	}
+}
+
+func TestGetLastUsedTimestampWithNoMatchingRoute(t *testing.T) {
+	table := RouteTable{}
+	config := defaultConfig()
+	if err := table.Init(config); err != nil {
+		t.Fatalf("table.Init(%#v): %s", config, err)
+	}
+
+	if _, found := table.GetLastUsedTimestamp(defaultRouteKey); found {
+		t.Errorf("got table.GetLastUsedTimetsamp(%#v) = (_, true), want = (_, false)", defaultRouteKey)
+	}
+}
+
+func TestSetLastUsedTimestamp(t *testing.T) {
+	clock := faketime.NewManualClock()
+	clock.Advance(10 * time.Second)
+
+	currentTime := clock.NowMonotonic()
+	validLastUsedTime := currentTime.Add(10 * time.Second)
+
+	tests := []struct {
+		name             string
+		lastUsedTime     tcpip.MonotonicTime
+		wantLastUsedTime tcpip.MonotonicTime
+	}{
+		{
+			name:             "valid timestamp",
+			lastUsedTime:     validLastUsedTime,
+			wantLastUsedTime: validLastUsedTime,
+		},
+		{
+			name:             "timestamp before",
+			lastUsedTime:     currentTime.Add(-5 * time.Second),
+			wantLastUsedTime: currentTime,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			table := RouteTable{}
+			config := defaultConfig(withClock(clock))
+			if err := table.Init(config); err != nil {
+				t.Fatalf("table.Init(%#v): %s", config, err)
+			}
+
+			route := table.NewInstalledRoute(inputNICID, defaultOutgoingInterfaces)
+
+			table.AddInstalledRoute(defaultRouteKey, route)
+
+			route.SetLastUsedTimestamp(test.lastUsedTime)
+
+			// Verify that the updated timestamp is actually reflected in the RouteTable.
+			timestamp, found := table.GetLastUsedTimestamp(defaultRouteKey)
+
+			if !found {
+				t.Fatalf("got table.GetLastUsedTimestamp(%#v) = (_, false_), want = (_, true)", defaultRouteKey)
+			}
+
+			if got, want := timestamp.Nanoseconds(), test.wantLastUsedTime.Nanoseconds(); got != want {
+				t.Errorf("got table.GetLastUsedTimestamp(%#v) = (%v, _), want (%v, _)", defaultRouteKey, got, want)
+			}
+		})
+	}
+}
+
 func TestMain(m *testing.M) {
 	refs.SetLeakMode(refs.LeaksPanic)
 	code := m.Run()

pkg/tcpip/tcpip.go

@@ -71,6 +71,11 @@ type MonotonicTime struct {
 	nanoseconds int64
 }
 
+// Nanoseconds returns the monotonic time in nanoseconds.
+func (mt MonotonicTime) Nanoseconds() int64 {
+	return mt.nanoseconds
+}
+
 // Before reports whether the monotonic clock reading mt is before u.
 func (mt MonotonicTime) Before(u MonotonicTime) bool {
 	return mt.nanoseconds < u.nanoseconds