Implement neighbor unreachability detection for ARP and NDP.

This change implements the Neighbor Unreachability Detection (NUD) state
machine, as per RFC 4861 [1]. The state machine operates on a single neighbor
in the local network. This requires the state machine to be implemented on each
entry of the neighbor table.

This change also adds, but does not expose, several APIs. The first API is for
performing basic operations on the neighbor table:
 - Create a static entry
 - List all entries
 - Delete all entries
 - Remove an entry by address

The second API is used for changing the NUD protocol constants on a per-NIC
basis to allow Neighbor Discovery to operate over links with widely varying
performance characteristics. See [RFC 4861 Section 10][2] for the list of
constants.

Finally, the last API is for allowing users to subscribe to NUD state changes.
See [RFC 4861 Appendix C][3] for the list of edges.

[1]: https://tools.ietf.org/html/rfc4861
[2]: https://tools.ietf.org/html/rfc4861#section-10
[3]: https://tools.ietf.org/html/rfc4861#appendix-C

Tests:
 pkg/tcpip/stack:stack_test
 - TestNeighborCacheAddStaticEntryThenOverflow
 - TestNeighborCacheClear
 - TestNeighborCacheClearThenOverflow
 - TestNeighborCacheConcurrent
 - TestNeighborCacheDuplicateStaticEntryWithDifferentLinkAddress
 - TestNeighborCacheDuplicateStaticEntryWithSameLinkAddress
 - TestNeighborCacheEntry
 - TestNeighborCacheEntryNoLinkAddress
 - TestNeighborCacheGetConfig
 - TestNeighborCacheKeepFrequentlyUsed
 - TestNeighborCacheNotifiesWaker
 - TestNeighborCacheOverflow
 - TestNeighborCacheOverwriteWithStaticEntryThenOverflow
 - TestNeighborCacheRemoveEntry
 - TestNeighborCacheRemoveEntryThenOverflow
 - TestNeighborCacheRemoveStaticEntry
 - TestNeighborCacheRemoveStaticEntryThenOverflow
 - TestNeighborCacheRemoveWaker
 - TestNeighborCacheReplace
 - TestNeighborCacheResolutionFailed
 - TestNeighborCacheResolutionTimeout
 - TestNeighborCacheSetConfig
 - TestNeighborCacheStaticResolution
 - TestEntryAddsAndClearsWakers
 - TestEntryDelayToProbe
 - TestEntryDelayToReachableWhenSolicitedOverrideConfirmation
 - TestEntryDelayToReachableWhenUpperLevelConfirmation
 - TestEntryDelayToStaleWhenConfirmationWithDifferentAddress
 - TestEntryDelayToStaleWhenProbeWithDifferentAddress
 - TestEntryFailedGetsDeleted
 - TestEntryIncompleteToFailed
 - TestEntryIncompleteToIncompleteDoesNotChangeUpdatedAt
 - TestEntryIncompleteToReachable
 - TestEntryIncompleteToReachableWithRouterFlag
 - TestEntryIncompleteToStale
 - TestEntryInitiallyUnknown
 - TestEntryProbeToFailed
 - TestEntryProbeToReachableWhenSolicitedConfirmationWithSameAddress
 - TestEntryProbeToReachableWhenSolicitedOverrideConfirmation
 - TestEntryProbeToStaleWhenConfirmationWithDifferentAddress
 - TestEntryProbeToStaleWhenProbeWithDifferentAddress
 - TestEntryReachableToStaleWhenConfirmationWithDifferentAddress
 - TestEntryReachableToStaleWhenConfirmationWithDifferentAddressAndOverride
 - TestEntryReachableToStaleWhenProbeWithDifferentAddress
 - TestEntryReachableToStaleWhenTimeout
 - TestEntryStaleToDelay
 - TestEntryStaleToReachableWhenSolicitedOverrideConfirmation
 - TestEntryStaleToStaleWhenOverrideConfirmation
 - TestEntryStaleToStaleWhenProbeUpdateAddress
 - TestEntryStaysDelayWhenOverrideConfirmationWithSameAddress
 - TestEntryStaysProbeWhenOverrideConfirmationWithSameAddress
 - TestEntryStaysReachableWhenConfirmationWithRouterFlag
 - TestEntryStaysReachableWhenProbeWithSameAddress
 - TestEntryStaysStaleWhenProbeWithSameAddress
 - TestEntryUnknownToIncomplete
 - TestEntryUnknownToStale
 - TestEntryUnknownToUnknownWhenConfirmationWithUnknownAddress

 pkg/tcpip/stack:stack_x_test
 - TestDefaultNUDConfigurations
 - TestNUDConfigurationFailsForNotSupported
 - TestNUDConfigurationsBaseReachableTime
 - TestNUDConfigurationsDelayFirstProbeTime
 - TestNUDConfigurationsMaxMulticastProbes
 - TestNUDConfigurationsMaxRandomFactor
 - TestNUDConfigurationsMaxUnicastProbes
 - TestNUDConfigurationsMinRandomFactor
 - TestNUDConfigurationsRetransmitTimer
 - TestNUDConfigurationsUnreachableTime
 - TestNUDStateReachableTime
 - TestNUDStateRecomputeReachableTime
 - TestSetNUDConfigurationFailsForBadNICID
 - TestSetNUDConfigurationFailsForNotSupported

[1]: https://tools.ietf.org/html/rfc4861
[2]: https://tools.ietf.org/html/rfc4861#section-10
[3]: https://tools.ietf.org/html/rfc4861#appendix-C

Updates #1889
Updates #1894
Updates #1895
Updates #1947
Updates #1948
Updates #1949
Updates #1950

PiperOrigin-RevId: 324070795

12 files changed, 6957 insertions, 17 deletions

diff --git a/pkg/tcpip/stack/BUILD b/pkg/tcpip/stack/BUILD
index 2b4455318..1c58bed2d 100644
--- a/pkg/tcpip/stack/BUILD
+++ b/pkg/tcpip/stack/BUILD
@@ -16,6 +16,18 @@ go_template_instance(
 )
 
 go_template_instance(
+    name = "neighbor_entry_list",
+    out = "neighbor_entry_list.go",
+    package = "stack",
+    prefix = "neighborEntry",
+    template = "//pkg/ilist:generic_list",
+    types = {
+        "Element": "*neighborEntry",
+        "Linker": "*neighborEntry",
+    },
+)
+
+go_template_instance(
     name = "packet_buffer_list",
     out = "packet_buffer_list.go",
     package = "stack",
@@ -53,7 +65,12 @@ go_library(
         "linkaddrcache.go",
         "linkaddrentry_list.go",
         "ndp.go",
+        "neighbor_cache.go",
+        "neighbor_entry.go",
+        "neighbor_entry_list.go",
+        "neighborstate_string.go",
         "nic.go",
+        "nud.go",
         "packet_buffer.go",
         "packet_buffer_list.go",
         "rand.go",
@@ -89,6 +106,7 @@ go_test(
     size = "medium",
     srcs = [
         "ndp_test.go",
+        "nud_test.go",
         "stack_test.go",
         "transport_demuxer_test.go",
         "transport_test.go",
@@ -118,8 +136,11 @@ go_test(
     name = "stack_test",
     size = "small",
     srcs = [
+        "fake_time_test.go",
         "forwarder_test.go",
         "linkaddrcache_test.go",
+        "neighbor_cache_test.go",
+        "neighbor_entry_test.go",
         "nic_test.go",
     ],
     library = ":stack",
@@ -129,5 +150,8 @@ go_test(
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
         "//pkg/tcpip/header",
+        "@com_github_dpjacques_clockwork//:go_default_library",
+        "@com_github_google_go_cmp//cmp:go_default_library",
+        "@com_github_google_go_cmp//cmp/cmpopts:go_default_library",
     ],
 )
diff --git a/pkg/tcpip/stack/fake_time_test.go b/pkg/tcpip/stack/fake_time_test.go
new file mode 100644
index 000000000..92c8cb534
--- /dev/null
+++ b/pkg/tcpip/stack/fake_time_test.go
@@ -0,0 +1,209 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"container/heap"
+	"sync"
+	"time"
+
+	"github.com/dpjacques/clockwork"
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
+
+type fakeClock struct {
+	clock clockwork.FakeClock
+
+	// mu protects the fields below.
+	mu sync.RWMutex
+
+	// times is min-heap of times. A heap is used for quick retrieval of the next
+	// upcoming time of scheduled work.
+	times *timeHeap
+
+	// waitGroups stores one WaitGroup for all work scheduled to execute at the
+	// same time via AfterFunc. This allows parallel execution of all functions
+	// passed to AfterFunc scheduled for the same time.
+	waitGroups map[time.Time]*sync.WaitGroup
+}
+
+func newFakeClock() *fakeClock {
+	return &fakeClock{
+		clock:      clockwork.NewFakeClock(),
+		times:      &timeHeap{},
+		waitGroups: make(map[time.Time]*sync.WaitGroup),
+	}
+}
+
+var _ tcpip.Clock = (*fakeClock)(nil)
+
+// NowNanoseconds implements tcpip.Clock.NowNanoseconds.
+func (fc *fakeClock) NowNanoseconds() int64 {
+	return fc.clock.Now().UnixNano()
+}
+
+// NowMonotonic implements tcpip.Clock.NowMonotonic.
+func (fc *fakeClock) NowMonotonic() int64 {
+	return fc.NowNanoseconds()
+}
+
+// AfterFunc implements tcpip.Clock.AfterFunc.
+func (fc *fakeClock) AfterFunc(d time.Duration, f func()) tcpip.Timer {
+	until := fc.clock.Now().Add(d)
+	wg := fc.addWait(until)
+	return &fakeTimer{
+		clock: fc,
+		until: until,
+		timer: fc.clock.AfterFunc(d, func() {
+			defer wg.Done()
+			f()
+		}),
+	}
+}
+
+// addWait adds an additional wait to the WaitGroup for parallel execution of
+// all work scheduled for t. Returns a reference to the WaitGroup modified.
+func (fc *fakeClock) addWait(t time.Time) *sync.WaitGroup {
+	fc.mu.RLock()
+	wg, ok := fc.waitGroups[t]
+	fc.mu.RUnlock()
+
+	if ok {
+		wg.Add(1)
+		return wg
+	}
+
+	fc.mu.Lock()
+	heap.Push(fc.times, t)
+	fc.mu.Unlock()
+
+	wg = &sync.WaitGroup{}
+	wg.Add(1)
+
+	fc.mu.Lock()
+	fc.waitGroups[t] = wg
+	fc.mu.Unlock()
+
+	return wg
+}
+
+// removeWait removes a wait from the WaitGroup for parallel execution of all
+// work scheduled for t.
+func (fc *fakeClock) removeWait(t time.Time) {
+	fc.mu.RLock()
+	defer fc.mu.RUnlock()
+
+	wg := fc.waitGroups[t]
+	wg.Done()
+}
+
+// advance executes all work that have been scheduled to execute within d from
+// the current fake time. Blocks until all work has completed execution.
+func (fc *fakeClock) advance(d time.Duration) {
+	// Block until all the work is done
+	until := fc.clock.Now().Add(d)
+	for {
+		fc.mu.Lock()
+		if fc.times.Len() == 0 {
+			fc.mu.Unlock()
+			return
+		}
+
+		t := heap.Pop(fc.times).(time.Time)
+		if t.After(until) {
+			// No work to do
+			heap.Push(fc.times, t)
+			fc.mu.Unlock()
+			return
+		}
+		fc.mu.Unlock()
+
+		diff := t.Sub(fc.clock.Now())
+		fc.clock.Advance(diff)
+
+		fc.mu.RLock()
+		wg := fc.waitGroups[t]
+		fc.mu.RUnlock()
+
+		wg.Wait()
+
+		fc.mu.Lock()
+		delete(fc.waitGroups, t)
+		fc.mu.Unlock()
+	}
+}
+
+type fakeTimer struct {
+	clock *fakeClock
+	timer clockwork.Timer
+
+	mu    sync.RWMutex
+	until time.Time
+}
+
+var _ tcpip.Timer = (*fakeTimer)(nil)
+
+// Reset implements tcpip.Timer.Reset.
+func (ft *fakeTimer) Reset(d time.Duration) {
+	if !ft.timer.Reset(d) {
+		return
+	}
+
+	ft.mu.Lock()
+	defer ft.mu.Unlock()
+
+	ft.clock.removeWait(ft.until)
+	ft.until = ft.clock.clock.Now().Add(d)
+	ft.clock.addWait(ft.until)
+}
+
+// Stop implements tcpip.Timer.Stop.
+func (ft *fakeTimer) Stop() bool {
+	if !ft.timer.Stop() {
+		return false
+	}
+
+	ft.mu.RLock()
+	defer ft.mu.RUnlock()
+
+	ft.clock.removeWait(ft.until)
+	return true
+}
+
+type timeHeap []time.Time
+
+var _ heap.Interface = (*timeHeap)(nil)
+
+func (h timeHeap) Len() int {
+	return len(h)
+}
+
+func (h timeHeap) Less(i, j int) bool {
+	return h[i].Before(h[j])
+}
+
+func (h timeHeap) Swap(i, j int) {
+	h[i], h[j] = h[j], h[i]
+}
+
+func (h *timeHeap) Push(x interface{}) {
+	*h = append(*h, x.(time.Time))
+}
+
+func (h *timeHeap) Pop() interface{} {
+	last := (*h)[len(*h)-1]
+	*h = (*h)[:len(*h)-1]
+	return last
+}
diff --git a/pkg/tcpip/stack/ndp.go b/pkg/tcpip/stack/ndp.go
index 9dce11a97..5174e639c 100644
--- a/pkg/tcpip/stack/ndp.go
+++ b/pkg/tcpip/stack/ndp.go
@@ -33,12 +33,6 @@ const (
 	// Default = 1 (from RFC 4862 section 5.1)
 	defaultDupAddrDetectTransmits = 1
 
-	// defaultRetransmitTimer is the default amount of time to wait between
-	// sending NDP Neighbor solicitation messages.
-	//
-	// Default = 1s (from RFC 4861 section 10).
-	defaultRetransmitTimer = time.Second
-
 	// defaultMaxRtrSolicitations is the default number of Router
 	// Solicitation messages to send when a NIC becomes enabled.
 	//
@@ -79,16 +73,6 @@ const (
 	// Default = true.
 	defaultAutoGenGlobalAddresses = true
 
-	// minimumRetransmitTimer is the minimum amount of time to wait between
-	// sending NDP Neighbor solicitation messages. Note, RFC 4861 does
-	// not impose a minimum Retransmit Timer, but we do here to make sure
-	// the messages are not sent all at once. We also come to this value
-	// because in the RetransmitTimer field of a Router Advertisement, a
-	// value of 0 means unspecified, so the smallest valid value is 1.
-	// Note, the unit of the RetransmitTimer field in the Router
-	// Advertisement is milliseconds.
-	minimumRetransmitTimer = time.Millisecond
-
 	// minimumRtrSolicitationInterval is the minimum amount of time to wait
 	// between sending Router Solicitation messages. This limit is imposed
 	// to make sure that Router Solicitation messages are not sent all at
diff --git a/pkg/tcpip/stack/neighbor_cache.go b/pkg/tcpip/stack/neighbor_cache.go
new file mode 100644
index 000000000..1d37716c2
--- /dev/null
+++ b/pkg/tcpip/stack/neighbor_cache.go
@@ -0,0 +1,335 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"fmt"
+	"time"
+
+	"gvisor.dev/gvisor/pkg/sleep"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
+
+const neighborCacheSize = 512 // max entries per interface
+
+// neighborCache maps IP addresses to link addresses. It uses the Least
+// Recently Used (LRU) eviction strategy to implement a bounded cache for
+// dynmically acquired entries. It contains the state machine and configuration
+// for running Neighbor Unreachability Detection (NUD).
+//
+// There are two types of entries in the neighbor cache:
+//  1. Dynamic entries are discovered automatically by neighbor discovery
+//     protocols (e.g. ARP, NDP). These protocols will attempt to reconfirm
+//     reachability with the device once the entry's state becomes Stale.
+//  2. Static entries are explicitly added by a user and have no expiration.
+//     Their state is always Static. The amount of static entries stored in the
+//     cache is unbounded.
+//
+// neighborCache implements NUDHandler.
+type neighborCache struct {
+	nic   *NIC
+	state *NUDState
+
+	// mu protects the fields below.
+	mu sync.RWMutex
+
+	cache   map[tcpip.Address]*neighborEntry
+	dynamic struct {
+		lru neighborEntryList
+
+		// count tracks the amount of dynamic entries in the cache. This is
+		// needed since static entries do not count towards the LRU cache
+		// eviction strategy.
+		count uint16
+	}
+}
+
+var _ NUDHandler = (*neighborCache)(nil)
+
+// getOrCreateEntry retrieves a cache entry associated with addr. The
+// returned entry is always refreshed in the cache (it is reachable via the
+// map, and its place is bumped in LRU).
+//
+// If a matching entry exists in the cache, it is returned. If no matching
+// entry exists and the cache is full, an existing entry is evicted via LRU,
+// reset to state incomplete, and returned. If no matching entry exists and the
+// cache is not full, a new entry with state incomplete is allocated and
+// returned.
+func (n *neighborCache) getOrCreateEntry(remoteAddr, localAddr tcpip.Address, linkRes LinkAddressResolver) *neighborEntry {
+	n.mu.Lock()
+	defer n.mu.Unlock()
+
+	if entry, ok := n.cache[remoteAddr]; ok {
+		entry.mu.RLock()
+		if entry.neigh.State != Static {
+			n.dynamic.lru.Remove(entry)
+			n.dynamic.lru.PushFront(entry)
+		}
+		entry.mu.RUnlock()
+		return entry
+	}
+
+	// The entry that needs to be created must be dynamic since all static
+	// entries are directly added to the cache via addStaticEntry.
+	entry := newNeighborEntry(n.nic, remoteAddr, localAddr, n.state, linkRes)
+	if n.dynamic.count == neighborCacheSize {
+		e := n.dynamic.lru.Back()
+		e.mu.Lock()
+
+		delete(n.cache, e.neigh.Addr)
+		n.dynamic.lru.Remove(e)
+		n.dynamic.count--
+
+		e.dispatchRemoveEventLocked()
+		e.setStateLocked(Unknown)
+		e.notifyWakersLocked()
+		e.mu.Unlock()
+	}
+	n.cache[remoteAddr] = entry
+	n.dynamic.lru.PushFront(entry)
+	n.dynamic.count++
+	return entry
+}
+
+// entry looks up the neighbor cache for translating address to link address
+// (e.g. IP -> MAC). If the LinkEndpoint requests address resolution and there
+// is a LinkAddressResolver registered with the network protocol, the cache
+// attempts to resolve the address and returns ErrWouldBlock. If a Waker is
+// provided, it will be notified when address resolution is complete (success
+// or not).
+//
+// If address resolution is required, ErrNoLinkAddress and a notification
+// channel is returned for the top level caller to block. Channel is closed
+// once address resolution is complete (success or not).
+func (n *neighborCache) entry(remoteAddr, localAddr tcpip.Address, linkRes LinkAddressResolver, w *sleep.Waker) (NeighborEntry, <-chan struct{}, *tcpip.Error) {
+	if linkRes != nil {
+		if linkAddr, ok := linkRes.ResolveStaticAddress(remoteAddr); ok {
+			e := NeighborEntry{
+				Addr:      remoteAddr,
+				LocalAddr: localAddr,
+				LinkAddr:  linkAddr,
+				State:     Static,
+				UpdatedAt: time.Now(),
+			}
+			return e, nil, nil
+		}
+	}
+
+	entry := n.getOrCreateEntry(remoteAddr, localAddr, linkRes)
+	entry.mu.Lock()
+	defer entry.mu.Unlock()
+
+	switch s := entry.neigh.State; s {
+	case Reachable, Static:
+		return entry.neigh, nil, nil
+
+	case Unknown, Incomplete, Stale, Delay, Probe:
+		entry.addWakerLocked(w)
+
+		if entry.done == nil {
+			// Address resolution needs to be initiated.
+			if linkRes == nil {
+				return entry.neigh, nil, tcpip.ErrNoLinkAddress
+			}
+			entry.done = make(chan struct{})
+		}
+
+		entry.handlePacketQueuedLocked()
+		return entry.neigh, entry.done, tcpip.ErrWouldBlock
+
+	case Failed:
+		return entry.neigh, nil, tcpip.ErrNoLinkAddress
+
+	default:
+		panic(fmt.Sprintf("Invalid cache entry state: %s", s))
+	}
+}
+
+// removeWaker removes a waker that has been added when link resolution for
+// addr was requested.
+func (n *neighborCache) removeWaker(addr tcpip.Address, waker *sleep.Waker) {
+	n.mu.Lock()
+	if entry, ok := n.cache[addr]; ok {
+		delete(entry.wakers, waker)
+	}
+	n.mu.Unlock()
+}
+
+// entries returns all entries in the neighbor cache.
+func (n *neighborCache) entries() []NeighborEntry {
+	entries := make([]NeighborEntry, 0, len(n.cache))
+	n.mu.RLock()
+	for _, entry := range n.cache {
+		entry.mu.RLock()
+		entries = append(entries, entry.neigh)
+		entry.mu.RUnlock()
+	}
+	n.mu.RUnlock()
+	return entries
+}
+
+// addStaticEntry adds a static entry to the neighbor cache, mapping an IP
+// address to a link address. If a dynamic entry exists in the neighbor cache
+// with the same address, it will be replaced with this static entry. If a
+// static entry exists with the same address but different link address, it
+// will be updated with the new link address. If a static entry exists with the
+// same address and link address, nothing will happen.
+func (n *neighborCache) addStaticEntry(addr tcpip.Address, linkAddr tcpip.LinkAddress) {
+	n.mu.Lock()
+	defer n.mu.Unlock()
+
+	if entry, ok := n.cache[addr]; ok {
+		entry.mu.Lock()
+		if entry.neigh.State != Static {
+			// Dynamic entry found with the same address.
+			n.dynamic.lru.Remove(entry)
+			n.dynamic.count--
+		} else if entry.neigh.LinkAddr == linkAddr {
+			// Static entry found with the same address and link address.
+			entry.mu.Unlock()
+			return
+		} else {
+			// Static entry found with the same address but different link address.
+			entry.neigh.LinkAddr = linkAddr
+			entry.dispatchChangeEventLocked(entry.neigh.State)
+			entry.mu.Unlock()
+			return
+		}
+
+		// Notify that resolution has been interrupted, just in case the entry was
+		// in the Incomplete or Probe state.
+		entry.dispatchRemoveEventLocked()
+		entry.setStateLocked(Unknown)
+		entry.notifyWakersLocked()
+		entry.mu.Unlock()
+	}
+
+	entry := newStaticNeighborEntry(n.nic, addr, linkAddr, n.state)
+	n.cache[addr] = entry
+}
+
+// removeEntryLocked removes the specified entry from the neighbor cache.
+func (n *neighborCache) removeEntryLocked(entry *neighborEntry) {
+	if entry.neigh.State != Static {
+		n.dynamic.lru.Remove(entry)
+		n.dynamic.count--
+	}
+	if entry.neigh.State != Failed {
+		entry.dispatchRemoveEventLocked()
+	}
+	entry.setStateLocked(Unknown)
+	entry.notifyWakersLocked()
+
+	delete(n.cache, entry.neigh.Addr)
+}
+
+// removeEntry removes a dynamic or static entry by address from the neighbor
+// cache. Returns true if the entry was found and deleted.
+func (n *neighborCache) removeEntry(addr tcpip.Address) bool {
+	n.mu.Lock()
+	defer n.mu.Unlock()
+
+	entry, ok := n.cache[addr]
+	if !ok {
+		return false
+	}
+
+	entry.mu.Lock()
+	defer entry.mu.Unlock()
+
+	n.removeEntryLocked(entry)
+	return true
+}
+
+// clear removes all dynamic and static entries from the neighbor cache.
+func (n *neighborCache) clear() {
+	n.mu.Lock()
+	defer n.mu.Unlock()
+
+	for _, entry := range n.cache {
+		entry.mu.Lock()
+		entry.dispatchRemoveEventLocked()
+		entry.setStateLocked(Unknown)
+		entry.notifyWakersLocked()
+		entry.mu.Unlock()
+	}
+
+	n.dynamic.lru = neighborEntryList{}
+	n.cache = make(map[tcpip.Address]*neighborEntry)
+	n.dynamic.count = 0
+}
+
+// config returns the NUD configuration.
+func (n *neighborCache) config() NUDConfigurations {
+	return n.state.Config()
+}
+
+// setConfig changes the NUD configuration.
+//
+// If config contains invalid NUD configuration values, it will be fixed to
+// use default values for the erroneous values.
+func (n *neighborCache) setConfig(config NUDConfigurations) {
+	config.resetInvalidFields()
+	n.state.SetConfig(config)
+}
+
+// HandleProbe implements NUDHandler.HandleProbe by following the logic defined
+// in RFC 4861 section 7.2.3. Validation of the probe is expected to be handled
+// by the caller.
+func (n *neighborCache) HandleProbe(remoteAddr, localAddr tcpip.Address, protocol tcpip.NetworkProtocolNumber, remoteLinkAddr tcpip.LinkAddress) {
+	entry := n.getOrCreateEntry(remoteAddr, localAddr, nil)
+	entry.mu.Lock()
+	entry.handleProbeLocked(remoteLinkAddr)
+	entry.mu.Unlock()
+}
+
+// HandleConfirmation implements NUDHandler.HandleConfirmation by following the
+// logic defined in RFC 4861 section 7.2.5.
+//
+// TODO(gvisor.dev/issue/2277): To protect against ARP poisoning and other
+// attacks against NDP functions, Secure Neighbor Discovery (SEND) Protocol
+// should be deployed where preventing access to the broadcast segment might
+// not be possible. SEND uses RSA key pairs to produce cryptographically
+// generated addresses, as defined in RFC 3972, Cryptographically Generated
+// Addresses (CGA). This ensures that the claimed source of an NDP message is
+// the owner of the claimed address.
+func (n *neighborCache) HandleConfirmation(addr tcpip.Address, linkAddr tcpip.LinkAddress, flags ReachabilityConfirmationFlags) {
+	n.mu.RLock()
+	entry, ok := n.cache[addr]
+	n.mu.RUnlock()
+	if ok {
+		entry.mu.Lock()
+		entry.handleConfirmationLocked(linkAddr, flags)
+		entry.mu.Unlock()
+	}
+	// The confirmation SHOULD be silently discarded if the recipient did not
+	// initiate any communication with the target. This is indicated if there is
+	// no matching entry for the remote address.
+}
+
+// HandleUpperLevelConfirmation implements
+// NUDHandler.HandleUpperLevelConfirmation by following the logic defined in
+// RFC 4861 section 7.3.1.
+func (n *neighborCache) HandleUpperLevelConfirmation(addr tcpip.Address) {
+	n.mu.RLock()
+	entry, ok := n.cache[addr]
+	n.mu.RUnlock()
+	if ok {
+		entry.mu.Lock()
+		entry.handleUpperLevelConfirmationLocked()
+		entry.mu.Unlock()
+	}
+}
diff --git a/pkg/tcpip/stack/neighbor_cache_test.go b/pkg/tcpip/stack/neighbor_cache_test.go
new file mode 100644
index 000000000..4cb2c9c6b
--- /dev/null
+++ b/pkg/tcpip/stack/neighbor_cache_test.go
@@ -0,0 +1,1752 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"math"
+	"math/rand"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+	"gvisor.dev/gvisor/pkg/sleep"
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
+
+const (
+	// entryStoreSize is the default number of entries that will be generated and
+	// added to the entry store. This number needs to be larger than the size of
+	// the neighbor cache to give ample opportunity for verifying behavior during
+	// cache overflows. Four times the size of the neighbor cache allows for
+	// three complete cache overflows.
+	entryStoreSize = 4 * neighborCacheSize
+
+	// typicalLatency is the typical latency for an ARP or NDP packet to travel
+	// to a router and back.
+	typicalLatency = time.Millisecond
+
+	// testEntryBroadcastAddr is a special address that indicates a packet should
+	// be sent to all nodes.
+	testEntryBroadcastAddr = tcpip.Address("broadcast")
+
+	// testEntryLocalAddr is the source address of neighbor probes.
+	testEntryLocalAddr = tcpip.Address("local_addr")
+
+	// testEntryBroadcastLinkAddr is a special link address sent back to
+	// multicast neighbor probes.
+	testEntryBroadcastLinkAddr = tcpip.LinkAddress("mac_broadcast")
+
+	// infiniteDuration indicates that a task will not occur in our lifetime.
+	infiniteDuration = time.Duration(math.MaxInt64)
+)
+
+// entryDiffOpts returns the options passed to cmp.Diff to compare neighbor
+// entries. The UpdatedAt field is ignored due to a lack of a deterministic
+// method to predict the time that an event will be dispatched.
+func entryDiffOpts() []cmp.Option {
+	return []cmp.Option{
+		cmpopts.IgnoreFields(NeighborEntry{}, "UpdatedAt"),
+	}
+}
+
+// entryDiffOptsWithSort is like entryDiffOpts but also includes an option to
+// sort slices of entries for cases where ordering must be ignored.
+func entryDiffOptsWithSort() []cmp.Option {
+	return []cmp.Option{
+		cmpopts.IgnoreFields(NeighborEntry{}, "UpdatedAt"),
+		cmpopts.SortSlices(func(a, b NeighborEntry) bool {
+			return strings.Compare(string(a.Addr), string(b.Addr)) < 0
+		}),
+	}
+}
+
+func newTestNeighborCache(nudDisp NUDDispatcher, config NUDConfigurations, clock tcpip.Clock) *neighborCache {
+	config.resetInvalidFields()
+	rng := rand.New(rand.NewSource(time.Now().UnixNano()))
+	return &neighborCache{
+		nic: &NIC{
+			stack: &Stack{
+				clock:   clock,
+				nudDisp: nudDisp,
+			},
+			id: 1,
+		},
+		state: NewNUDState(config, rng),
+		cache: make(map[tcpip.Address]*neighborEntry, neighborCacheSize),
+	}
+}
+
+// testEntryStore contains a set of IP to NeighborEntry mappings.
+type testEntryStore struct {
+	mu         sync.RWMutex
+	entriesMap map[tcpip.Address]NeighborEntry
+}
+
+func toAddress(i int) tcpip.Address {
+	buf := new(bytes.Buffer)
+	binary.Write(buf, binary.BigEndian, uint8(1))
+	binary.Write(buf, binary.BigEndian, uint8(0))
+	binary.Write(buf, binary.BigEndian, uint16(i))
+	return tcpip.Address(buf.String())
+}
+
+func toLinkAddress(i int) tcpip.LinkAddress {
+	buf := new(bytes.Buffer)
+	binary.Write(buf, binary.BigEndian, uint8(1))
+	binary.Write(buf, binary.BigEndian, uint8(0))
+	binary.Write(buf, binary.BigEndian, uint32(i))
+	return tcpip.LinkAddress(buf.String())
+}
+
+// newTestEntryStore returns a testEntryStore pre-populated with entries.
+func newTestEntryStore() *testEntryStore {
+	store := &testEntryStore{
+		entriesMap: make(map[tcpip.Address]NeighborEntry),
+	}
+	for i := 0; i < entryStoreSize; i++ {
+		addr := toAddress(i)
+		linkAddr := toLinkAddress(i)
+
+		store.entriesMap[addr] = NeighborEntry{
+			Addr:      addr,
+			LocalAddr: testEntryLocalAddr,
+			LinkAddr:  linkAddr,
+		}
+	}
+	return store
+}
+
+// size returns the number of entries in the store.
+func (s *testEntryStore) size() int {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return len(s.entriesMap)
+}
+
+// entry returns the entry at index i. Returns an empty entry and false if i is
+// out of bounds.
+func (s *testEntryStore) entry(i int) (NeighborEntry, bool) {
+	return s.entryByAddr(toAddress(i))
+}
+
+// entryByAddr returns the entry matching addr for situations when the index is
+// not available. Returns an empty entry and false if no entries match addr.
+func (s *testEntryStore) entryByAddr(addr tcpip.Address) (NeighborEntry, bool) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	entry, ok := s.entriesMap[addr]
+	return entry, ok
+}
+
+// entries returns all entries in the store.
+func (s *testEntryStore) entries() []NeighborEntry {
+	entries := make([]NeighborEntry, 0, len(s.entriesMap))
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	for i := 0; i < entryStoreSize; i++ {
+		addr := toAddress(i)
+		if entry, ok := s.entriesMap[addr]; ok {
+			entries = append(entries, entry)
+		}
+	}
+	return entries
+}
+
+// set modifies the link addresses of an entry.
+func (s *testEntryStore) set(i int, linkAddr tcpip.LinkAddress) {
+	addr := toAddress(i)
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if entry, ok := s.entriesMap[addr]; ok {
+		entry.LinkAddr = linkAddr
+		s.entriesMap[addr] = entry
+	}
+}
+
+// testNeighborResolver implements LinkAddressResolver to emulate sending a
+// neighbor probe.
+type testNeighborResolver struct {
+	clock                tcpip.Clock
+	neigh                *neighborCache
+	entries              *testEntryStore
+	delay                time.Duration
+	onLinkAddressRequest func()
+}
+
+var _ LinkAddressResolver = (*testNeighborResolver)(nil)
+
+func (r *testNeighborResolver) LinkAddressRequest(addr, localAddr tcpip.Address, linkAddr tcpip.LinkAddress, linkEP LinkEndpoint) *tcpip.Error {
+	// Delay handling the request to emulate network latency.
+	r.clock.AfterFunc(r.delay, func() {
+		r.fakeRequest(addr)
+	})
+
+	// Execute post address resolution action, if available.
+	if f := r.onLinkAddressRequest; f != nil {
+		f()
+	}
+	return nil
+}
+
+// fakeRequest emulates handling a response for a link address request.
+func (r *testNeighborResolver) fakeRequest(addr tcpip.Address) {
+	if entry, ok := r.entries.entryByAddr(addr); ok {
+		r.neigh.HandleConfirmation(addr, entry.LinkAddr, ReachabilityConfirmationFlags{
+			Solicited: true,
+			Override:  false,
+			IsRouter:  false,
+		})
+	}
+}
+
+func (*testNeighborResolver) ResolveStaticAddress(addr tcpip.Address) (tcpip.LinkAddress, bool) {
+	if addr == testEntryBroadcastAddr {
+		return testEntryBroadcastLinkAddr, true
+	}
+	return "", false
+}
+
+func (*testNeighborResolver) LinkAddressProtocol() tcpip.NetworkProtocolNumber {
+	return 0
+}
+
+type entryEvent struct {
+	nicID    tcpip.NICID
+	address  tcpip.Address
+	linkAddr tcpip.LinkAddress
+	state    NeighborState
+}
+
+func TestNeighborCacheGetConfig(t *testing.T) {
+	nudDisp := testNUDDispatcher{}
+	c := DefaultNUDConfigurations()
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, c, clock)
+
+	if got, want := neigh.config(), c; got != want {
+		t.Errorf("got neigh.config() = %+v, want = %+v", got, want)
+	}
+
+	// No events should have been dispatched.
+	nudDisp.mu.Lock()
+	defer nudDisp.mu.Unlock()
+	if diff := cmp.Diff(nudDisp.events, []testEntryEventInfo(nil)); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+func TestNeighborCacheSetConfig(t *testing.T) {
+	nudDisp := testNUDDispatcher{}
+	c := DefaultNUDConfigurations()
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, c, clock)
+
+	c.MinRandomFactor = 1
+	c.MaxRandomFactor = 1
+	neigh.setConfig(c)
+
+	if got, want := neigh.config(), c; got != want {
+		t.Errorf("got neigh.config() = %+v, want = %+v", got, want)
+	}
+
+	// No events should have been dispatched.
+	nudDisp.mu.Lock()
+	defer nudDisp.mu.Unlock()
+	if diff := cmp.Diff(nudDisp.events, []testEntryEventInfo(nil)); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+func TestNeighborCacheEntry(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	nudDisp := testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, c, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	entry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+	_, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+	if err != tcpip.ErrWouldBlock {
+		t.Errorf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+
+	clock.advance(typicalLatency)
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...)
+	nudDisp.events = nil
+	nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	if _, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil); err != nil {
+		t.Fatalf("unexpected error from neigh.entry(%s, %s, _, nil): %s", entry.Addr, entry.LocalAddr, err)
+	}
+
+	// No more events should have been dispatched.
+	nudDisp.mu.Lock()
+	defer nudDisp.mu.Unlock()
+	if diff := cmp.Diff(nudDisp.events, []testEntryEventInfo(nil)); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+// TestNeighborCacheEntryNoLinkAddress verifies calling entry() without a
+// LinkAddressResolver returns ErrNoLinkAddress.
+func TestNeighborCacheEntryNoLinkAddress(t *testing.T) {
+	nudDisp := testNUDDispatcher{}
+	c := DefaultNUDConfigurations()
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, c, clock)
+	store := newTestEntryStore()
+
+	entry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+	_, _, err := neigh.entry(entry.Addr, entry.LocalAddr, nil, nil)
+	if err != tcpip.ErrNoLinkAddress {
+		t.Errorf("got neigh.entry(%s, %s, nil, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrNoLinkAddress)
+	}
+
+	// No events should have been dispatched.
+	nudDisp.mu.Lock()
+	defer nudDisp.mu.Unlock()
+	if diff := cmp.Diff(nudDisp.events, []testEntryEventInfo(nil)); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+func TestNeighborCacheRemoveEntry(t *testing.T) {
+	config := DefaultNUDConfigurations()
+
+	nudDisp := testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	entry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+	_, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+	if err != tcpip.ErrWouldBlock {
+		t.Errorf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+
+	clock.advance(typicalLatency)
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...)
+	nudDisp.events = nil
+	nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	neigh.removeEntry(entry.Addr)
+
+	{
+		wantEvents := []testEntryEventInfo{
+			{
+				EventType: entryTestRemoved,
+				NICID:     1,
+				Addr:      entry.Addr,
+				LinkAddr:  entry.LinkAddr,
+				State:     Reachable,
+			},
+		}
+		nudDisp.mu.Lock()
+		diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...)
+		nudDisp.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	if _, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil); err != tcpip.ErrWouldBlock {
+		t.Errorf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+}
+
+type testContext struct {
+	clock   *fakeClock
+	neigh   *neighborCache
+	store   *testEntryStore
+	linkRes *testNeighborResolver
+	nudDisp *testNUDDispatcher
+}
+
+func newTestContext(c NUDConfigurations) testContext {
+	nudDisp := &testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(nudDisp, c, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	return testContext{
+		clock:   clock,
+		neigh:   neigh,
+		store:   store,
+		linkRes: linkRes,
+		nudDisp: nudDisp,
+	}
+}
+
+type overflowOptions struct {
+	startAtEntryIndex int
+	wantStaticEntries []NeighborEntry
+}
+
+func (c *testContext) overflowCache(opts overflowOptions) error {
+	// Fill the neighbor cache to capacity to verify the LRU eviction strategy is
+	// working properly after the entry removal.
+	for i := opts.startAtEntryIndex; i < c.store.size(); i++ {
+		// Add a new entry
+		entry, ok := c.store.entry(i)
+		if !ok {
+			return fmt.Errorf("c.store.entry(%d) not found", i)
+		}
+		if _, _, err := c.neigh.entry(entry.Addr, entry.LocalAddr, c.linkRes, nil); err != tcpip.ErrWouldBlock {
+			return fmt.Errorf("got c.neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+		}
+		c.clock.advance(c.neigh.config().RetransmitTimer)
+
+		var wantEvents []testEntryEventInfo
+
+		// When beyond the full capacity, the cache will evict an entry as per the
+		// LRU eviction strategy. Note that the number of static entries should not
+		// affect the total number of dynamic entries that can be added.
+		if i >= neighborCacheSize+opts.startAtEntryIndex {
+			removedEntry, ok := c.store.entry(i - neighborCacheSize)
+			if !ok {
+				return fmt.Errorf("store.entry(%d) not found", i-neighborCacheSize)
+			}
+			wantEvents = append(wantEvents, testEntryEventInfo{
+				EventType: entryTestRemoved,
+				NICID:     1,
+				Addr:      removedEntry.Addr,
+				LinkAddr:  removedEntry.LinkAddr,
+				State:     Reachable,
+			})
+		}
+
+		wantEvents = append(wantEvents, testEntryEventInfo{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			State:     Incomplete,
+		}, testEntryEventInfo{
+			EventType: entryTestChanged,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		})
+
+		c.nudDisp.mu.Lock()
+		diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+		c.nudDisp.events = nil
+		c.nudDisp.mu.Unlock()
+		if diff != "" {
+			return fmt.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	// Expect to find only the most recent entries. The order of entries reported
+	// by entries() is undeterministic, so entries have to be sorted before
+	// comparison.
+	wantUnsortedEntries := opts.wantStaticEntries
+	for i := c.store.size() - neighborCacheSize; i < c.store.size(); i++ {
+		entry, ok := c.store.entry(i)
+		if !ok {
+			return fmt.Errorf("c.store.entry(%d) not found", i)
+		}
+		wantEntry := NeighborEntry{
+			Addr:      entry.Addr,
+			LocalAddr: entry.LocalAddr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		}
+		wantUnsortedEntries = append(wantUnsortedEntries, wantEntry)
+	}
+
+	if diff := cmp.Diff(c.neigh.entries(), wantUnsortedEntries, entryDiffOptsWithSort()...); diff != "" {
+		return fmt.Errorf("neighbor entries mismatch (-got, +want):\n%s", diff)
+	}
+
+	// No more events should have been dispatched.
+	c.nudDisp.mu.Lock()
+	defer c.nudDisp.mu.Unlock()
+	if diff := cmp.Diff(c.nudDisp.events, []testEntryEventInfo(nil)); diff != "" {
+		return fmt.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	return nil
+}
+
+// TestNeighborCacheOverflow verifies that the LRU cache eviction strategy
+// respects the dynamic entry count.
+func TestNeighborCacheOverflow(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	// Stay in Reachable so the cache can overflow
+	config.BaseReachableTime = infiniteDuration
+	config.MinRandomFactor = 1
+	config.MaxRandomFactor = 1
+
+	c := newTestContext(config)
+	opts := overflowOptions{
+		startAtEntryIndex: 0,
+	}
+	if err := c.overflowCache(opts); err != nil {
+		t.Errorf("c.overflowCache(%+v): %s", opts, err)
+	}
+}
+
+// TestNeighborCacheRemoveEntryThenOverflow verifies that the LRU cache
+// eviction strategy respects the dynamic entry count when an entry is removed.
+func TestNeighborCacheRemoveEntryThenOverflow(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	// Stay in Reachable so the cache can overflow
+	config.BaseReachableTime = infiniteDuration
+	config.MinRandomFactor = 1
+	config.MaxRandomFactor = 1
+
+	c := newTestContext(config)
+
+	// Add a dynamic entry
+	entry, ok := c.store.entry(0)
+	if !ok {
+		t.Fatalf("c.store.entry(0) not found")
+	}
+	_, _, err := c.neigh.entry(entry.Addr, entry.LocalAddr, c.linkRes, nil)
+	if err != tcpip.ErrWouldBlock {
+		t.Errorf("got c.neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	c.clock.advance(c.neigh.config().RetransmitTimer)
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		},
+	}
+	c.nudDisp.mu.Lock()
+	diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+	c.nudDisp.events = nil
+	c.nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	// Remove the entry
+	c.neigh.removeEntry(entry.Addr)
+
+	{
+		wantEvents := []testEntryEventInfo{
+			{
+				EventType: entryTestRemoved,
+				NICID:     1,
+				Addr:      entry.Addr,
+				LinkAddr:  entry.LinkAddr,
+				State:     Reachable,
+			},
+		}
+		c.nudDisp.mu.Lock()
+		diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+		c.nudDisp.events = nil
+		c.nudDisp.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	opts := overflowOptions{
+		startAtEntryIndex: 0,
+	}
+	if err := c.overflowCache(opts); err != nil {
+		t.Errorf("c.overflowCache(%+v): %s", opts, err)
+	}
+}
+
+// TestNeighborCacheDuplicateStaticEntryWithSameLinkAddress verifies that
+// adding a duplicate static entry with the same link address does not dispatch
+// any events.
+func TestNeighborCacheDuplicateStaticEntryWithSameLinkAddress(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	c := newTestContext(config)
+
+	// Add a static entry
+	entry, ok := c.store.entry(0)
+	if !ok {
+		t.Fatalf("c.store.entry(0) not found")
+	}
+	staticLinkAddr := entry.LinkAddr + "static"
+	c.neigh.addStaticEntry(entry.Addr, staticLinkAddr)
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  staticLinkAddr,
+			State:     Static,
+		},
+	}
+	c.nudDisp.mu.Lock()
+	diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+	c.nudDisp.events = nil
+	c.nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	// Remove the static entry that was just added
+	c.neigh.addStaticEntry(entry.Addr, staticLinkAddr)
+
+	// No more events should have been dispatched.
+	c.nudDisp.mu.Lock()
+	defer c.nudDisp.mu.Unlock()
+	if diff := cmp.Diff(c.nudDisp.events, []testEntryEventInfo(nil)); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+// TestNeighborCacheDuplicateStaticEntryWithDifferentLinkAddress verifies that
+// adding a duplicate static entry with a different link address dispatches a
+// change event.
+func TestNeighborCacheDuplicateStaticEntryWithDifferentLinkAddress(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	c := newTestContext(config)
+
+	// Add a static entry
+	entry, ok := c.store.entry(0)
+	if !ok {
+		t.Fatalf("c.store.entry(0) not found")
+	}
+	staticLinkAddr := entry.LinkAddr + "static"
+	c.neigh.addStaticEntry(entry.Addr, staticLinkAddr)
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  staticLinkAddr,
+			State:     Static,
+		},
+	}
+	c.nudDisp.mu.Lock()
+	diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+	c.nudDisp.events = nil
+	c.nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	// Add a duplicate entry with a different link address
+	staticLinkAddr += "duplicate"
+	c.neigh.addStaticEntry(entry.Addr, staticLinkAddr)
+	{
+		wantEvents := []testEntryEventInfo{
+			{
+				EventType: entryTestChanged,
+				NICID:     1,
+				Addr:      entry.Addr,
+				LinkAddr:  staticLinkAddr,
+				State:     Static,
+			},
+		}
+		c.nudDisp.mu.Lock()
+		defer c.nudDisp.mu.Unlock()
+		if diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+			t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+}
+
+// TestNeighborCacheRemoveStaticEntryThenOverflow verifies that the LRU cache
+// eviction strategy respects the dynamic entry count when a static entry is
+// added then removed. In this case, the dynamic entry count shouldn't have
+// been touched.
+func TestNeighborCacheRemoveStaticEntryThenOverflow(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	// Stay in Reachable so the cache can overflow
+	config.BaseReachableTime = infiniteDuration
+	config.MinRandomFactor = 1
+	config.MaxRandomFactor = 1
+
+	c := newTestContext(config)
+
+	// Add a static entry
+	entry, ok := c.store.entry(0)
+	if !ok {
+		t.Fatalf("c.store.entry(0) not found")
+	}
+	staticLinkAddr := entry.LinkAddr + "static"
+	c.neigh.addStaticEntry(entry.Addr, staticLinkAddr)
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  staticLinkAddr,
+			State:     Static,
+		},
+	}
+	c.nudDisp.mu.Lock()
+	diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+	c.nudDisp.events = nil
+	c.nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	// Remove the static entry that was just added
+	c.neigh.removeEntry(entry.Addr)
+	{
+		wantEvents := []testEntryEventInfo{
+			{
+				EventType: entryTestRemoved,
+				NICID:     1,
+				Addr:      entry.Addr,
+				LinkAddr:  staticLinkAddr,
+				State:     Static,
+			},
+		}
+		c.nudDisp.mu.Lock()
+		diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+		c.nudDisp.events = nil
+		c.nudDisp.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	opts := overflowOptions{
+		startAtEntryIndex: 0,
+	}
+	if err := c.overflowCache(opts); err != nil {
+		t.Errorf("c.overflowCache(%+v): %s", opts, err)
+	}
+}
+
+// TestNeighborCacheOverwriteWithStaticEntryThenOverflow verifies that the LRU
+// cache eviction strategy keeps count of the dynamic entry count when an entry
+// is overwritten by a static entry. Static entries should not count towards
+// the size of the LRU cache.
+func TestNeighborCacheOverwriteWithStaticEntryThenOverflow(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	// Stay in Reachable so the cache can overflow
+	config.BaseReachableTime = infiniteDuration
+	config.MinRandomFactor = 1
+	config.MaxRandomFactor = 1
+
+	c := newTestContext(config)
+
+	// Add a dynamic entry
+	entry, ok := c.store.entry(0)
+	if !ok {
+		t.Fatalf("c.store.entry(0) not found")
+	}
+	_, _, err := c.neigh.entry(entry.Addr, entry.LocalAddr, c.linkRes, nil)
+	if err != tcpip.ErrWouldBlock {
+		t.Errorf("got c.neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	c.clock.advance(typicalLatency)
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		},
+	}
+	c.nudDisp.mu.Lock()
+	diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+	c.nudDisp.events = nil
+	c.nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	// Override the entry with a static one using the same address
+	staticLinkAddr := entry.LinkAddr + "static"
+	c.neigh.addStaticEntry(entry.Addr, staticLinkAddr)
+	{
+		wantEvents := []testEntryEventInfo{
+			{
+				EventType: entryTestRemoved,
+				NICID:     1,
+				Addr:      entry.Addr,
+				LinkAddr:  entry.LinkAddr,
+				State:     Reachable,
+			},
+			{
+				EventType: entryTestAdded,
+				NICID:     1,
+				Addr:      entry.Addr,
+				LinkAddr:  staticLinkAddr,
+				State:     Static,
+			},
+		}
+		c.nudDisp.mu.Lock()
+		diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+		c.nudDisp.events = nil
+		c.nudDisp.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	opts := overflowOptions{
+		startAtEntryIndex: 1,
+		wantStaticEntries: []NeighborEntry{
+			{
+				Addr:      entry.Addr,
+				LocalAddr: "", // static entries don't need a local address
+				LinkAddr:  staticLinkAddr,
+				State:     Static,
+			},
+		},
+	}
+	if err := c.overflowCache(opts); err != nil {
+		t.Errorf("c.overflowCache(%+v): %s", opts, err)
+	}
+}
+
+func TestNeighborCacheNotifiesWaker(t *testing.T) {
+	config := DefaultNUDConfigurations()
+
+	nudDisp := testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	w := sleep.Waker{}
+	s := sleep.Sleeper{}
+	const wakerID = 1
+	s.AddWaker(&w, wakerID)
+
+	entry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+	_, doneCh, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, &w)
+	if err != tcpip.ErrWouldBlock {
+		t.Fatalf("got neigh.entry(%s, %s, _, _ = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	if doneCh == nil {
+		t.Fatalf("expected done channel from neigh.entry(%s, %s, _, _)", entry.Addr, entry.LocalAddr)
+	}
+	clock.advance(typicalLatency)
+
+	select {
+	case <-doneCh:
+	default:
+		t.Fatal("expected notification from done channel")
+	}
+
+	id, ok := s.Fetch(false /* block */)
+	if !ok {
+		t.Errorf("expected waker to be notified after neigh.entry(%s, %s, _, _)", entry.Addr, entry.LocalAddr)
+	}
+	if id != wakerID {
+		t.Errorf("got s.Fetch(false) = %d, want = %d", id, wakerID)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	defer nudDisp.mu.Unlock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+func TestNeighborCacheRemoveWaker(t *testing.T) {
+	config := DefaultNUDConfigurations()
+
+	nudDisp := testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	w := sleep.Waker{}
+	s := sleep.Sleeper{}
+	const wakerID = 1
+	s.AddWaker(&w, wakerID)
+
+	entry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+	_, doneCh, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, &w)
+	if err != tcpip.ErrWouldBlock {
+		t.Fatalf("got neigh.entry(%s, %s, _, _) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	if doneCh == nil {
+		t.Fatalf("expected done channel from neigh.entry(%s, %s, _, _)", entry.Addr, entry.LocalAddr)
+	}
+
+	// Remove the waker before the neighbor cache has the opportunity to send a
+	// notification.
+	neigh.removeWaker(entry.Addr, &w)
+	clock.advance(typicalLatency)
+
+	select {
+	case <-doneCh:
+	default:
+		t.Fatal("expected notification from done channel")
+	}
+
+	if id, ok := s.Fetch(false /* block */); ok {
+		t.Errorf("unexpected notification from waker with id %d", id)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	defer nudDisp.mu.Unlock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+func TestNeighborCacheAddStaticEntryThenOverflow(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	// Stay in Reachable so the cache can overflow
+	config.BaseReachableTime = infiniteDuration
+	config.MinRandomFactor = 1
+	config.MaxRandomFactor = 1
+
+	c := newTestContext(config)
+
+	entry, ok := c.store.entry(0)
+	if !ok {
+		t.Fatalf("c.store.entry(0) not found")
+	}
+	c.neigh.addStaticEntry(entry.Addr, entry.LinkAddr)
+	e, _, err := c.neigh.entry(entry.Addr, "", nil, nil)
+	if err != nil {
+		t.Errorf("unexpected error from c.neigh.entry(%s, \"\", nil nil): %s", entry.Addr, err)
+	}
+	want := NeighborEntry{
+		Addr:      entry.Addr,
+		LocalAddr: "", // static entries don't need a local address
+		LinkAddr:  entry.LinkAddr,
+		State:     Static,
+	}
+	if diff := cmp.Diff(e, want, entryDiffOpts()...); diff != "" {
+		t.Errorf("c.neigh.entry(%s, \"\", nil, nil) mismatch (-got, +want):\n%s", entry.Addr, diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Static,
+		},
+	}
+	c.nudDisp.mu.Lock()
+	diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+	c.nudDisp.events = nil
+	c.nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	opts := overflowOptions{
+		startAtEntryIndex: 1,
+		wantStaticEntries: []NeighborEntry{
+			{
+				Addr:      entry.Addr,
+				LocalAddr: "", // static entries don't need a local address
+				LinkAddr:  entry.LinkAddr,
+				State:     Static,
+			},
+		},
+	}
+	if err := c.overflowCache(opts); err != nil {
+		t.Errorf("c.overflowCache(%+v): %s", opts, err)
+	}
+}
+
+func TestNeighborCacheClear(t *testing.T) {
+	config := DefaultNUDConfigurations()
+
+	nudDisp := testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	// Add a dynamic entry.
+	entry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+	_, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+	if err != tcpip.ErrWouldBlock {
+		t.Errorf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	clock.advance(typicalLatency)
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...)
+	nudDisp.events = nil
+	nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	// Add a static entry.
+	neigh.addStaticEntry(entryTestAddr1, entryTestLinkAddr1)
+
+	{
+		wantEvents := []testEntryEventInfo{
+			{
+				EventType: entryTestAdded,
+				NICID:     1,
+				Addr:      entryTestAddr1,
+				LinkAddr:  entryTestLinkAddr1,
+				State:     Static,
+			},
+		}
+		nudDisp.mu.Lock()
+		diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...)
+		nudDisp.events = nil
+		nudDisp.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	// Clear shoud remove both dynamic and static entries.
+	neigh.clear()
+
+	// Remove events dispatched from clear() have no deterministic order so they
+	// need to be sorted beforehand.
+	wantUnsortedEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestRemoved,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		},
+		{
+			EventType: entryTestRemoved,
+			NICID:     1,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Static,
+		},
+	}
+	nudDisp.mu.Lock()
+	defer nudDisp.mu.Unlock()
+	if diff := cmp.Diff(nudDisp.events, wantUnsortedEvents, eventDiffOptsWithSort()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+// TestNeighborCacheClearThenOverflow verifies that the LRU cache eviction
+// strategy keeps count of the dynamic entry count when all entries are
+// cleared.
+func TestNeighborCacheClearThenOverflow(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	// Stay in Reachable so the cache can overflow
+	config.BaseReachableTime = infiniteDuration
+	config.MinRandomFactor = 1
+	config.MaxRandomFactor = 1
+
+	c := newTestContext(config)
+
+	// Add a dynamic entry
+	entry, ok := c.store.entry(0)
+	if !ok {
+		t.Fatalf("c.store.entry(0) not found")
+	}
+	_, _, err := c.neigh.entry(entry.Addr, entry.LocalAddr, c.linkRes, nil)
+	if err != tcpip.ErrWouldBlock {
+		t.Errorf("got c.neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	c.clock.advance(typicalLatency)
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     1,
+			Addr:      entry.Addr,
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     1,
+			Addr:      entry.Addr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		},
+	}
+	c.nudDisp.mu.Lock()
+	diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+	c.nudDisp.events = nil
+	c.nudDisp.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+
+	// Clear the cache.
+	c.neigh.clear()
+	{
+		wantEvents := []testEntryEventInfo{
+			{
+				EventType: entryTestRemoved,
+				NICID:     1,
+				Addr:      entry.Addr,
+				LinkAddr:  entry.LinkAddr,
+				State:     Reachable,
+			},
+		}
+		c.nudDisp.mu.Lock()
+		diff := cmp.Diff(c.nudDisp.events, wantEvents, eventDiffOpts()...)
+		c.nudDisp.events = nil
+		c.nudDisp.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	opts := overflowOptions{
+		startAtEntryIndex: 0,
+	}
+	if err := c.overflowCache(opts); err != nil {
+		t.Errorf("c.overflowCache(%+v): %s", opts, err)
+	}
+}
+
+func TestNeighborCacheKeepFrequentlyUsed(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	// Stay in Reachable so the cache can overflow
+	config.BaseReachableTime = infiniteDuration
+	config.MinRandomFactor = 1
+	config.MaxRandomFactor = 1
+
+	nudDisp := testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	frequentlyUsedEntry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+
+	// The following logic is very similar to overflowCache, but
+	// periodically refreshes the frequently used entry.
+
+	// Fill the neighbor cache to capacity
+	for i := 0; i < neighborCacheSize; i++ {
+		entry, ok := store.entry(i)
+		if !ok {
+			t.Fatalf("store.entry(%d) not found", i)
+		}
+		_, doneCh, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+		if err != tcpip.ErrWouldBlock {
+			t.Errorf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+		}
+		clock.advance(typicalLatency)
+		select {
+		case <-doneCh:
+		default:
+			t.Fatalf("expected notification from done channel returned by neigh.entry(%s, %s, _, nil)", entry.Addr, entry.LocalAddr)
+		}
+		wantEvents := []testEntryEventInfo{
+			{
+				EventType: entryTestAdded,
+				NICID:     1,
+				Addr:      entry.Addr,
+				State:     Incomplete,
+			},
+			{
+				EventType: entryTestChanged,
+				NICID:     1,
+				Addr:      entry.Addr,
+				LinkAddr:  entry.LinkAddr,
+				State:     Reachable,
+			},
+		}
+		nudDisp.mu.Lock()
+		diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...)
+		nudDisp.events = nil
+		nudDisp.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	// Keep adding more entries
+	for i := neighborCacheSize; i < store.size(); i++ {
+		// Periodically refresh the frequently used entry
+		if i%(neighborCacheSize/2) == 0 {
+			_, _, err := neigh.entry(frequentlyUsedEntry.Addr, frequentlyUsedEntry.LocalAddr, linkRes, nil)
+			if err != nil {
+				t.Errorf("unexpected error from neigh.entry(%s, %s, _, nil): %s", frequentlyUsedEntry.Addr, frequentlyUsedEntry.LocalAddr, err)
+			}
+		}
+
+		entry, ok := store.entry(i)
+		if !ok {
+			t.Fatalf("store.entry(%d) not found", i)
+		}
+		_, doneCh, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+		if err != tcpip.ErrWouldBlock {
+			t.Errorf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+		}
+		clock.advance(typicalLatency)
+		select {
+		case <-doneCh:
+		default:
+			t.Fatalf("expected notification from done channel returned by neigh.entry(%s, %s, _, nil)", entry.Addr, entry.LocalAddr)
+		}
+
+		// An entry should have been removed, as per the LRU eviction strategy
+		removedEntry, ok := store.entry(i - neighborCacheSize + 1)
+		if !ok {
+			t.Fatalf("store.entry(%d) not found", i-neighborCacheSize+1)
+		}
+		wantEvents := []testEntryEventInfo{
+			{
+				EventType: entryTestRemoved,
+				NICID:     1,
+				Addr:      removedEntry.Addr,
+				LinkAddr:  removedEntry.LinkAddr,
+				State:     Reachable,
+			},
+			{
+				EventType: entryTestAdded,
+				NICID:     1,
+				Addr:      entry.Addr,
+				State:     Incomplete,
+			},
+			{
+				EventType: entryTestChanged,
+				NICID:     1,
+				Addr:      entry.Addr,
+				LinkAddr:  entry.LinkAddr,
+				State:     Reachable,
+			},
+		}
+		nudDisp.mu.Lock()
+		diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...)
+		nudDisp.events = nil
+		nudDisp.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	// Expect to find only the frequently used entry and the most recent entries.
+	// The order of entries reported by entries() is undeterministic, so entries
+	// have to be sorted before comparison.
+	wantUnsortedEntries := []NeighborEntry{
+		{
+			Addr:      frequentlyUsedEntry.Addr,
+			LocalAddr: frequentlyUsedEntry.LocalAddr,
+			LinkAddr:  frequentlyUsedEntry.LinkAddr,
+			State:     Reachable,
+		},
+	}
+
+	for i := store.size() - neighborCacheSize + 1; i < store.size(); i++ {
+		entry, ok := store.entry(i)
+		if !ok {
+			t.Fatalf("store.entry(%d) not found", i)
+		}
+		wantEntry := NeighborEntry{
+			Addr:      entry.Addr,
+			LocalAddr: entry.LocalAddr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		}
+		wantUnsortedEntries = append(wantUnsortedEntries, wantEntry)
+	}
+
+	if diff := cmp.Diff(neigh.entries(), wantUnsortedEntries, entryDiffOptsWithSort()...); diff != "" {
+		t.Errorf("neighbor entries mismatch (-got, +want):\n%s", diff)
+	}
+
+	// No more events should have been dispatched.
+	nudDisp.mu.Lock()
+	defer nudDisp.mu.Unlock()
+	if diff := cmp.Diff(nudDisp.events, []testEntryEventInfo(nil)); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+func TestNeighborCacheConcurrent(t *testing.T) {
+	const concurrentProcesses = 16
+
+	config := DefaultNUDConfigurations()
+
+	nudDisp := testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	storeEntries := store.entries()
+	for _, entry := range storeEntries {
+		var wg sync.WaitGroup
+		for r := 0; r < concurrentProcesses; r++ {
+			wg.Add(1)
+			go func(entry NeighborEntry) {
+				defer wg.Done()
+				e, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+				if err != nil && err != tcpip.ErrWouldBlock {
+					t.Errorf("got neigh.entry(%s, %s, _, nil) = (%+v, _, %s), want (_, _, nil) or (_, _, %s)", entry.Addr, entry.LocalAddr, e, err, tcpip.ErrWouldBlock)
+				}
+			}(entry)
+		}
+
+		// Wait for all gorountines to send a request
+		wg.Wait()
+
+		// Process all the requests for a single entry concurrently
+		clock.advance(typicalLatency)
+	}
+
+	// All goroutines add in the same order and add more values than can fit in
+	// the cache. Our eviction strategy requires that the last entries are
+	// present, up to the size of the neighbor cache, and the rest are missing.
+	// The order of entries reported by entries() is undeterministic, so entries
+	// have to be sorted before comparison.
+	var wantUnsortedEntries []NeighborEntry
+	for i := store.size() - neighborCacheSize; i < store.size(); i++ {
+		entry, ok := store.entry(i)
+		if !ok {
+			t.Errorf("store.entry(%d) not found", i)
+		}
+		wantEntry := NeighborEntry{
+			Addr:      entry.Addr,
+			LocalAddr: entry.LocalAddr,
+			LinkAddr:  entry.LinkAddr,
+			State:     Reachable,
+		}
+		wantUnsortedEntries = append(wantUnsortedEntries, wantEntry)
+	}
+
+	if diff := cmp.Diff(neigh.entries(), wantUnsortedEntries, entryDiffOptsWithSort()...); diff != "" {
+		t.Errorf("neighbor entries mismatch (-got, +want):\n%s", diff)
+	}
+}
+
+func TestNeighborCacheReplace(t *testing.T) {
+	config := DefaultNUDConfigurations()
+
+	nudDisp := testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	// Add an entry
+	entry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+	_, doneCh, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+	if err != tcpip.ErrWouldBlock {
+		t.Fatalf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	clock.advance(typicalLatency)
+	select {
+	case <-doneCh:
+	default:
+		t.Fatalf("expected notification from done channel returned by neigh.entry(%s, %s, _, nil)", entry.Addr, entry.LocalAddr)
+	}
+
+	// Verify the entry exists
+	e, doneCh, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+	if err != nil {
+		t.Errorf("unexpected error from neigh.entry(%s, %s, _, nil): %s", entry.Addr, entry.LocalAddr, err)
+	}
+	if doneCh != nil {
+		t.Errorf("unexpected done channel from neigh.entry(%s, %s, _, nil): %v", entry.Addr, entry.LocalAddr, doneCh)
+	}
+	if t.Failed() {
+		t.FailNow()
+	}
+	want := NeighborEntry{
+		Addr:      entry.Addr,
+		LocalAddr: entry.LocalAddr,
+		LinkAddr:  entry.LinkAddr,
+		State:     Reachable,
+	}
+	if diff := cmp.Diff(e, want, entryDiffOpts()...); diff != "" {
+		t.Errorf("neigh.entry(%s, %s, _, nil) mismatch (-got, +want):\n%s", entry.Addr, entry.LinkAddr, diff)
+	}
+
+	// Notify of a link address change
+	var updatedLinkAddr tcpip.LinkAddress
+	{
+		entry, ok := store.entry(1)
+		if !ok {
+			t.Fatalf("store.entry(1) not found")
+		}
+		updatedLinkAddr = entry.LinkAddr
+	}
+	store.set(0, updatedLinkAddr)
+	neigh.HandleConfirmation(entry.Addr, updatedLinkAddr, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  true,
+		IsRouter:  false,
+	})
+
+	// Requesting the entry again should start address resolution
+	{
+		_, doneCh, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+		if err != tcpip.ErrWouldBlock {
+			t.Fatalf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+		}
+		clock.advance(config.DelayFirstProbeTime + typicalLatency)
+		select {
+		case <-doneCh:
+		default:
+			t.Fatalf("expected notification from done channel returned by neigh.entry(%s, %s, _, nil)", entry.Addr, entry.LocalAddr)
+		}
+	}
+
+	// Verify the entry's new link address
+	{
+		e, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+		clock.advance(typicalLatency)
+		if err != nil {
+			t.Errorf("unexpected error from neigh.entry(%s, %s, _, nil): %s", entry.Addr, entry.LocalAddr, err)
+		}
+		want = NeighborEntry{
+			Addr:      entry.Addr,
+			LocalAddr: entry.LocalAddr,
+			LinkAddr:  updatedLinkAddr,
+			State:     Reachable,
+		}
+		if diff := cmp.Diff(e, want, entryDiffOpts()...); diff != "" {
+			t.Errorf("neigh.entry(%s, %s, _, nil) mismatch (-got, +want):\n%s", entry.Addr, entry.LocalAddr, diff)
+		}
+	}
+}
+
+func TestNeighborCacheResolutionFailed(t *testing.T) {
+	config := DefaultNUDConfigurations()
+
+	nudDisp := testNUDDispatcher{}
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(&nudDisp, config, clock)
+	store := newTestEntryStore()
+
+	var requestCount uint32
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+		onLinkAddressRequest: func() {
+			atomic.AddUint32(&requestCount, 1)
+		},
+	}
+
+	// First, sanity check that resolution is working
+	entry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+	if _, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil); err != tcpip.ErrWouldBlock {
+		t.Fatalf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	clock.advance(typicalLatency)
+	got, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+	if err != nil {
+		t.Fatalf("unexpected error from neigh.entry(%s, %s, _, nil): %s", entry.Addr, entry.LocalAddr, err)
+	}
+	want := NeighborEntry{
+		Addr:      entry.Addr,
+		LocalAddr: entry.LocalAddr,
+		LinkAddr:  entry.LinkAddr,
+		State:     Reachable,
+	}
+	if diff := cmp.Diff(got, want, entryDiffOpts()...); diff != "" {
+		t.Errorf("neigh.entry(%s, %s, _, nil) mismatch (-got, +want):\n%s", entry.Addr, entry.LocalAddr, diff)
+	}
+
+	// Verify that address resolution for an unknown address returns ErrNoLinkAddress
+	before := atomic.LoadUint32(&requestCount)
+
+	entry.Addr += "2"
+	if _, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil); err != tcpip.ErrWouldBlock {
+		t.Fatalf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	waitFor := config.DelayFirstProbeTime + typicalLatency*time.Duration(config.MaxMulticastProbes)
+	clock.advance(waitFor)
+	if _, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil); err != tcpip.ErrNoLinkAddress {
+		t.Fatalf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrNoLinkAddress)
+	}
+
+	maxAttempts := neigh.config().MaxUnicastProbes
+	if got, want := atomic.LoadUint32(&requestCount)-before, maxAttempts; got != want {
+		t.Errorf("got link address request count = %d, want = %d", got, want)
+	}
+}
+
+// TestNeighborCacheResolutionTimeout simulates sending MaxMulticastProbes
+// probes and not retrieving a confirmation before the duration defined by
+// MaxMulticastProbes * RetransmitTimer.
+func TestNeighborCacheResolutionTimeout(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	config.RetransmitTimer = time.Millisecond // small enough to cause timeout
+
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(nil, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   time.Minute, // large enough to cause timeout
+	}
+
+	entry, ok := store.entry(0)
+	if !ok {
+		t.Fatalf("store.entry(0) not found")
+	}
+	if _, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil); err != tcpip.ErrWouldBlock {
+		t.Fatalf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+	}
+	waitFor := config.RetransmitTimer * time.Duration(config.MaxMulticastProbes)
+	clock.advance(waitFor)
+	if _, _, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil); err != tcpip.ErrNoLinkAddress {
+		t.Fatalf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrNoLinkAddress)
+	}
+}
+
+// TestNeighborCacheStaticResolution checks that static link addresses are
+// resolved immediately and don't send resolution requests.
+func TestNeighborCacheStaticResolution(t *testing.T) {
+	config := DefaultNUDConfigurations()
+	clock := newFakeClock()
+	neigh := newTestNeighborCache(nil, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   typicalLatency,
+	}
+
+	got, _, err := neigh.entry(testEntryBroadcastAddr, testEntryLocalAddr, linkRes, nil)
+	if err != nil {
+		t.Fatalf("unexpected error from neigh.entry(%s, %s, _, nil): %s", testEntryBroadcastAddr, testEntryLocalAddr, err)
+	}
+	want := NeighborEntry{
+		Addr:      testEntryBroadcastAddr,
+		LocalAddr: testEntryLocalAddr,
+		LinkAddr:  testEntryBroadcastLinkAddr,
+		State:     Static,
+	}
+	if diff := cmp.Diff(got, want, entryDiffOpts()...); diff != "" {
+		t.Errorf("neigh.entry(%s, %s, _, nil) mismatch (-got, +want):\n%s", testEntryBroadcastAddr, testEntryLocalAddr, diff)
+	}
+}
+
+func BenchmarkCacheClear(b *testing.B) {
+	b.StopTimer()
+	config := DefaultNUDConfigurations()
+	clock := &tcpip.StdClock{}
+	neigh := newTestNeighborCache(nil, config, clock)
+	store := newTestEntryStore()
+	linkRes := &testNeighborResolver{
+		clock:   clock,
+		neigh:   neigh,
+		entries: store,
+		delay:   0,
+	}
+
+	// Clear for every possible size of the cache
+	for cacheSize := 0; cacheSize < neighborCacheSize; cacheSize++ {
+		// Fill the neighbor cache to capacity.
+		for i := 0; i < cacheSize; i++ {
+			entry, ok := store.entry(i)
+			if !ok {
+				b.Fatalf("store.entry(%d) not found", i)
+			}
+			_, doneCh, err := neigh.entry(entry.Addr, entry.LocalAddr, linkRes, nil)
+			if err != tcpip.ErrWouldBlock {
+				b.Fatalf("got neigh.entry(%s, %s, _, nil) = %v, want = %s", entry.Addr, entry.LocalAddr, err, tcpip.ErrWouldBlock)
+			}
+			if doneCh != nil {
+				<-doneCh
+			}
+		}
+
+		b.StartTimer()
+		neigh.clear()
+		b.StopTimer()
+	}
+}
diff --git a/pkg/tcpip/stack/neighbor_entry.go b/pkg/tcpip/stack/neighbor_entry.go
new file mode 100644
index 000000000..0068cacb8
--- /dev/null
+++ b/pkg/tcpip/stack/neighbor_entry.go
@@ -0,0 +1,482 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"gvisor.dev/gvisor/pkg/sleep"
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
+
+// NeighborEntry describes a neighboring device in the local network.
+type NeighborEntry struct {
+	Addr      tcpip.Address
+	LocalAddr tcpip.Address
+	LinkAddr  tcpip.LinkAddress
+	State     NeighborState
+	UpdatedAt time.Time
+}
+
+// NeighborState defines the state of a NeighborEntry within the Neighbor
+// Unreachability Detection state machine, as per RFC 4861 section 7.3.2.
+type NeighborState uint8
+
+const (
+	// Unknown means reachability has not been verified yet. This is the initial
+	// state of entries that have been created automatically by the Neighbor
+	// Unreachability Detection state machine.
+	Unknown NeighborState = iota
+	// Incomplete means that there is an outstanding request to resolve the
+	// address.
+	Incomplete
+	// Reachable means the path to the neighbor is functioning properly for both
+	// receive and transmit paths.
+	Reachable
+	// Stale means reachability to the neighbor is unknown, but packets are still
+	// able to be transmitted to the possibly stale link address.
+	Stale
+	// Delay means reachability to the neighbor is unknown and pending
+	// confirmation from an upper-level protocol like TCP, but packets are still
+	// able to be transmitted to the possibly stale link address.
+	Delay
+	// Probe means a reachability confirmation is actively being sought by
+	// periodically retransmitting reachability probes until a reachability
+	// confirmation is received, or until the max amount of probes has been sent.
+	Probe
+	// Static describes entries that have been explicitly added by the user. They
+	// do not expire and are not deleted until explicitly removed.
+	Static
+	// Failed means traffic should not be sent to this neighbor since attempts of
+	// reachability have returned inconclusive.
+	Failed
+)
+
+// neighborEntry implements a neighbor entry's individual node behavior, as per
+// RFC 4861 section 7.3.3. Neighbor Unreachability Detection operates in
+// parallel with the sending of packets to a neighbor, necessitating the
+// entry's lock to be acquired for all operations.
+type neighborEntry struct {
+	neighborEntryEntry
+
+	nic      *NIC
+	protocol tcpip.NetworkProtocolNumber
+
+	// linkRes provides the functionality to send reachability probes, used in
+	// Neighbor Unreachability Detection.
+	linkRes LinkAddressResolver
+
+	// nudState points to the Neighbor Unreachability Detection configuration.
+	nudState *NUDState
+
+	// mu protects the fields below.
+	mu sync.RWMutex
+
+	neigh NeighborEntry
+
+	// wakers is a set of waiters for address resolution result. Anytime state
+	// transitions out of incomplete these waiters are notified. It is nil iff
+	// address resolution is ongoing and no clients are waiting for the result.
+	wakers map[*sleep.Waker]struct{}
+
+	// done is used to allow callers to wait on address resolution. It is nil
+	// iff nudState is not Reachable and address resolution is not yet in
+	// progress.
+	done chan struct{}
+
+	isRouter bool
+	job      *tcpip.Job
+}
+
+// newNeighborEntry creates a neighbor cache entry starting at the default
+// state, Unknown. Transition out of Unknown by calling either
+// `handlePacketQueuedLocked` or `handleProbeLocked` on the newly created
+// neighborEntry.
+func newNeighborEntry(nic *NIC, remoteAddr tcpip.Address, localAddr tcpip.Address, nudState *NUDState, linkRes LinkAddressResolver) *neighborEntry {
+	return &neighborEntry{
+		nic:      nic,
+		linkRes:  linkRes,
+		nudState: nudState,
+		neigh: NeighborEntry{
+			Addr:      remoteAddr,
+			LocalAddr: localAddr,
+			State:     Unknown,
+		},
+	}
+}
+
+// newStaticNeighborEntry creates a neighbor cache entry starting at the Static
+// state. The entry can only transition out of Static by directly calling
+// `setStateLocked`.
+func newStaticNeighborEntry(nic *NIC, addr tcpip.Address, linkAddr tcpip.LinkAddress, state *NUDState) *neighborEntry {
+	if nic.stack.nudDisp != nil {
+		nic.stack.nudDisp.OnNeighborAdded(nic.id, addr, linkAddr, Static, time.Now())
+	}
+	return &neighborEntry{
+		nic:      nic,
+		nudState: state,
+		neigh: NeighborEntry{
+			Addr:      addr,
+			LinkAddr:  linkAddr,
+			State:     Static,
+			UpdatedAt: time.Now(),
+		},
+	}
+}
+
+// addWaker adds w to the list of wakers waiting for address resolution.
+// Assumes the entry has already been appropriately locked.
+func (e *neighborEntry) addWakerLocked(w *sleep.Waker) {
+	if w == nil {
+		return
+	}
+	if e.wakers == nil {
+		e.wakers = make(map[*sleep.Waker]struct{})
+	}
+	e.wakers[w] = struct{}{}
+}
+
+// notifyWakersLocked notifies those waiting for address resolution, whether it
+// succeeded or failed. Assumes the entry has already been appropriately locked.
+func (e *neighborEntry) notifyWakersLocked() {
+	for w := range e.wakers {
+		w.Assert()
+	}
+	e.wakers = nil
+	if ch := e.done; ch != nil {
+		close(ch)
+		e.done = nil
+	}
+}
+
+// dispatchAddEventLocked signals to stack's NUD Dispatcher that the entry has
+// been added.
+func (e *neighborEntry) dispatchAddEventLocked(nextState NeighborState) {
+	if nudDisp := e.nic.stack.nudDisp; nudDisp != nil {
+		nudDisp.OnNeighborAdded(e.nic.id, e.neigh.Addr, e.neigh.LinkAddr, nextState, time.Now())
+	}
+}
+
+// dispatchChangeEventLocked signals to stack's NUD Dispatcher that the entry
+// has changed state or link-layer address.
+func (e *neighborEntry) dispatchChangeEventLocked(nextState NeighborState) {
+	if nudDisp := e.nic.stack.nudDisp; nudDisp != nil {
+		nudDisp.OnNeighborChanged(e.nic.id, e.neigh.Addr, e.neigh.LinkAddr, nextState, time.Now())
+	}
+}
+
+// dispatchRemoveEventLocked signals to stack's NUD Dispatcher that the entry
+// has been removed.
+func (e *neighborEntry) dispatchRemoveEventLocked() {
+	if nudDisp := e.nic.stack.nudDisp; nudDisp != nil {
+		nudDisp.OnNeighborRemoved(e.nic.id, e.neigh.Addr, e.neigh.LinkAddr, e.neigh.State, time.Now())
+	}
+}
+
+// setStateLocked transitions the entry to the specified state immediately.
+//
+// Follows the logic defined in RFC 4861 section 7.3.3.
+//
+// e.mu MUST be locked.
+func (e *neighborEntry) setStateLocked(next NeighborState) {
+	// Cancel the previously scheduled action, if there is one. Entries in
+	// Unknown, Stale, or Static state do not have scheduled actions.
+	if timer := e.job; timer != nil {
+		timer.Cancel()
+	}
+
+	prev := e.neigh.State
+	e.neigh.State = next
+	e.neigh.UpdatedAt = time.Now()
+	config := e.nudState.Config()
+
+	switch next {
+	case Incomplete:
+		var retryCounter uint32
+		var sendMulticastProbe func()
+
+		sendMulticastProbe = func() {
+			if retryCounter == config.MaxMulticastProbes {
+				// "If no Neighbor Advertisement is received after
+				// MAX_MULTICAST_SOLICIT solicitations, address resolution has failed.
+				// The sender MUST return ICMP destination unreachable indications with
+				// code 3 (Address Unreachable) for each packet queued awaiting address
+				// resolution." - RFC 4861 section 7.2.2
+				//
+				// There is no need to send an ICMP destination unreachable indication
+				// since the failure to resolve the address is expected to only occur
+				// on this node. Thus, redirecting traffic is currently not supported.
+				//
+				// "If the error occurs on a node other than the node originating the
+				// packet, an ICMP error message is generated. If the error occurs on
+				// the originating node, an implementation is not required to actually
+				// create and send an ICMP error packet to the source, as long as the
+				// upper-layer sender is notified through an appropriate mechanism
+				// (e.g. return value from a procedure call). Note, however, that an
+				// implementation may find it convenient in some cases to return errors
+				// to the sender by taking the offending packet, generating an ICMP
+				// error message, and then delivering it (locally) through the generic
+				// error-handling routines.' - RFC 4861 section 2.1
+				e.dispatchRemoveEventLocked()
+				e.setStateLocked(Failed)
+				return
+			}
+
+			if err := e.linkRes.LinkAddressRequest(e.neigh.Addr, e.neigh.LocalAddr, "", e.nic.linkEP); err != nil {
+				// There is no need to log the error here; the NUD implementation may
+				// assume a working link. A valid link should be the responsibility of
+				// the NIC/stack.LinkEndpoint.
+				e.dispatchRemoveEventLocked()
+				e.setStateLocked(Failed)
+				return
+			}
+
+			retryCounter++
+			e.job = e.nic.stack.newJob(&e.mu, sendMulticastProbe)
+			e.job.Schedule(config.RetransmitTimer)
+		}
+
+		sendMulticastProbe()
+
+	case Reachable:
+		e.job = e.nic.stack.newJob(&e.mu, func() {
+			e.dispatchChangeEventLocked(Stale)
+			e.setStateLocked(Stale)
+		})
+		e.job.Schedule(e.nudState.ReachableTime())
+
+	case Delay:
+		e.job = e.nic.stack.newJob(&e.mu, func() {
+			e.dispatchChangeEventLocked(Probe)
+			e.setStateLocked(Probe)
+		})
+		e.job.Schedule(config.DelayFirstProbeTime)
+
+	case Probe:
+		var retryCounter uint32
+		var sendUnicastProbe func()
+
+		sendUnicastProbe = func() {
+			if retryCounter == config.MaxUnicastProbes {
+				e.dispatchRemoveEventLocked()
+				e.setStateLocked(Failed)
+				return
+			}
+
+			if err := e.linkRes.LinkAddressRequest(e.neigh.Addr, e.neigh.LocalAddr, e.neigh.LinkAddr, e.nic.linkEP); err != nil {
+				e.dispatchRemoveEventLocked()
+				e.setStateLocked(Failed)
+				return
+			}
+
+			retryCounter++
+			if retryCounter == config.MaxUnicastProbes {
+				e.dispatchRemoveEventLocked()
+				e.setStateLocked(Failed)
+				return
+			}
+
+			e.job = e.nic.stack.newJob(&e.mu, sendUnicastProbe)
+			e.job.Schedule(config.RetransmitTimer)
+		}
+
+		sendUnicastProbe()
+
+	case Failed:
+		e.notifyWakersLocked()
+		e.job = e.nic.stack.newJob(&e.mu, func() {
+			e.nic.neigh.removeEntryLocked(e)
+		})
+		e.job.Schedule(config.UnreachableTime)
+
+	case Unknown, Stale, Static:
+		// Do nothing
+
+	default:
+		panic(fmt.Sprintf("Invalid state transition from %q to %q", prev, next))
+	}
+}
+
+// handlePacketQueuedLocked advances the state machine according to a packet
+// being queued for outgoing transmission.
+//
+// Follows the logic defined in RFC 4861 section 7.3.3.
+func (e *neighborEntry) handlePacketQueuedLocked() {
+	switch e.neigh.State {
+	case Unknown:
+		e.dispatchAddEventLocked(Incomplete)
+		e.setStateLocked(Incomplete)
+
+	case Stale:
+		e.dispatchChangeEventLocked(Delay)
+		e.setStateLocked(Delay)
+
+	case Incomplete, Reachable, Delay, Probe, Static, Failed:
+		// Do nothing
+
+	default:
+		panic(fmt.Sprintf("Invalid cache entry state: %s", e.neigh.State))
+	}
+}
+
+// handleProbeLocked processes an incoming neighbor probe (e.g. ARP request or
+// Neighbor Solicitation for ARP or NDP, respectively).
+//
+// Follows the logic defined in RFC 4861 section 7.2.3.
+func (e *neighborEntry) handleProbeLocked(remoteLinkAddr tcpip.LinkAddress) {
+	// Probes MUST be silently discarded if the target address is tentative, does
+	// not exist, or not bound to the NIC as per RFC 4861 section 7.2.3. These
+	// checks MUST be done by the NetworkEndpoint.
+
+	switch e.neigh.State {
+	case Unknown, Incomplete, Failed:
+		e.neigh.LinkAddr = remoteLinkAddr
+		e.dispatchAddEventLocked(Stale)
+		e.setStateLocked(Stale)
+		e.notifyWakersLocked()
+
+	case Reachable, Delay, Probe:
+		if e.neigh.LinkAddr != remoteLinkAddr {
+			e.neigh.LinkAddr = remoteLinkAddr
+			e.dispatchChangeEventLocked(Stale)
+			e.setStateLocked(Stale)
+		}
+
+	case Stale:
+		if e.neigh.LinkAddr != remoteLinkAddr {
+			e.neigh.LinkAddr = remoteLinkAddr
+			e.dispatchChangeEventLocked(Stale)
+		}
+
+	case Static:
+		// Do nothing
+
+	default:
+		panic(fmt.Sprintf("Invalid cache entry state: %s", e.neigh.State))
+	}
+}
+
+// handleConfirmationLocked processes an incoming neighbor confirmation
+// (e.g. ARP reply or Neighbor Advertisement for ARP or NDP, respectively).
+//
+// Follows the state machine defined by RFC 4861 section 7.2.5.
+//
+// TODO(gvisor.dev/issue/2277): To protect against ARP poisoning and other
+// attacks against NDP functions, Secure Neighbor Discovery (SEND) Protocol
+// should be deployed where preventing access to the broadcast segment might
+// not be possible. SEND uses RSA key pairs to produce Cryptographically
+// Generated Addresses (CGA), as defined in RFC 3972. This ensures that the
+// claimed source of an NDP message is the owner of the claimed address.
+func (e *neighborEntry) handleConfirmationLocked(linkAddr tcpip.LinkAddress, flags ReachabilityConfirmationFlags) {
+	switch e.neigh.State {
+	case Incomplete:
+		if len(linkAddr) == 0 {
+			// "If the link layer has addresses and no Target Link-Layer Address
+			// option is included, the receiving node SHOULD silently discard the
+			// received advertisement." - RFC 4861 section 7.2.5
+			break
+		}
+
+		e.neigh.LinkAddr = linkAddr
+		if flags.Solicited {
+			e.dispatchChangeEventLocked(Reachable)
+			e.setStateLocked(Reachable)
+		} else {
+			e.dispatchChangeEventLocked(Stale)
+			e.setStateLocked(Stale)
+		}
+		e.isRouter = flags.IsRouter
+		e.notifyWakersLocked()
+
+		// "Note that the Override flag is ignored if the entry is in the
+		// INCOMPLETE state." - RFC 4861 section 7.2.5
+
+	case Reachable, Stale, Delay, Probe:
+		sameLinkAddr := e.neigh.LinkAddr == linkAddr
+
+		if !sameLinkAddr {
+			if !flags.Override {
+				if e.neigh.State == Reachable {
+					e.dispatchChangeEventLocked(Stale)
+					e.setStateLocked(Stale)
+				}
+				break
+			}
+
+			e.neigh.LinkAddr = linkAddr
+
+			if !flags.Solicited {
+				if e.neigh.State != Stale {
+					e.dispatchChangeEventLocked(Stale)
+					e.setStateLocked(Stale)
+				} else {
+					// Notify the LinkAddr change, even though NUD state hasn't changed.
+					e.dispatchChangeEventLocked(e.neigh.State)
+				}
+				break
+			}
+		}
+
+		if flags.Solicited && (flags.Override || sameLinkAddr) {
+			if e.neigh.State != Reachable {
+				e.dispatchChangeEventLocked(Reachable)
+			}
+			// Set state to Reachable again to refresh timers.
+			e.setStateLocked(Reachable)
+			e.notifyWakersLocked()
+		}
+
+		if e.isRouter && !flags.IsRouter {
+			// "In those cases where the IsRouter flag changes from TRUE to FALSE as
+			// a result of this update, the node MUST remove that router from the
+			// Default Router List and update the Destination Cache entries for all
+			// destinations using that neighbor as a router as specified in Section
+			// 7.3.3.  This is needed to detect when a node that is used as a router
+			// stops forwarding packets due to being configured as a host."
+			//  - RFC 4861 section 7.2.5
+			e.nic.mu.Lock()
+			e.nic.mu.ndp.invalidateDefaultRouter(e.neigh.Addr)
+			e.nic.mu.Unlock()
+		}
+		e.isRouter = flags.IsRouter
+
+	case Unknown, Failed, Static:
+		// Do nothing
+
+	default:
+		panic(fmt.Sprintf("Invalid cache entry state: %s", e.neigh.State))
+	}
+}
+
+// handleUpperLevelConfirmationLocked processes an incoming upper-level protocol
+// (e.g. TCP acknowledgements) reachability confirmation.
+func (e *neighborEntry) handleUpperLevelConfirmationLocked() {
+	switch e.neigh.State {
+	case Reachable, Stale, Delay, Probe:
+		if e.neigh.State != Reachable {
+			e.dispatchChangeEventLocked(Reachable)
+			// Set state to Reachable again to refresh timers.
+		}
+		e.setStateLocked(Reachable)
+
+	case Unknown, Incomplete, Failed, Static:
+		// Do nothing
+
+	default:
+		panic(fmt.Sprintf("Invalid cache entry state: %s", e.neigh.State))
+	}
+}
diff --git a/pkg/tcpip/stack/neighbor_entry_test.go b/pkg/tcpip/stack/neighbor_entry_test.go
new file mode 100644
index 000000000..08c9ccd25
--- /dev/null
+++ b/pkg/tcpip/stack/neighbor_entry_test.go
@@ -0,0 +1,2770 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"fmt"
+	"math"
+	"math/rand"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+	"gvisor.dev/gvisor/pkg/sleep"
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
+
+const (
+	entryTestNetNumber tcpip.NetworkProtocolNumber = math.MaxUint32
+
+	entryTestNICID tcpip.NICID = 1
+	entryTestAddr1             = tcpip.Address("\x00\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
+	entryTestAddr2             = tcpip.Address("\x00\x0a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02")
+
+	entryTestLinkAddr1 = tcpip.LinkAddress("\x0a\x00\x00\x00\x00\x01")
+	entryTestLinkAddr2 = tcpip.LinkAddress("\x0a\x00\x00\x00\x00\x02")
+
+	// entryTestNetDefaultMTU is the MTU, in bytes, used throughout the tests,
+	// except where another value is explicitly used. It is chosen to match the
+	// MTU of loopback interfaces on Linux systems.
+	entryTestNetDefaultMTU = 65536
+)
+
+// eventDiffOpts are the options passed to cmp.Diff to compare entry events.
+// The UpdatedAt field is ignored due to a lack of a deterministic method to
+// predict the time that an event will be dispatched.
+func eventDiffOpts() []cmp.Option {
+	return []cmp.Option{
+		cmpopts.IgnoreFields(testEntryEventInfo{}, "UpdatedAt"),
+	}
+}
+
+// eventDiffOptsWithSort is like eventDiffOpts but also includes an option to
+// sort slices of events for cases where ordering must be ignored.
+func eventDiffOptsWithSort() []cmp.Option {
+	return []cmp.Option{
+		cmpopts.IgnoreFields(testEntryEventInfo{}, "UpdatedAt"),
+		cmpopts.SortSlices(func(a, b testEntryEventInfo) bool {
+			return strings.Compare(string(a.Addr), string(b.Addr)) < 0
+		}),
+	}
+}
+
+// The following unit tests exercise every state transition and verify its
+// behavior with RFC 4681.
+//
+// | From       | To         | Cause                                      | Action          | Event   |
+// | ========== | ========== | ========================================== | =============== | ======= |
+// | Unknown    | Unknown    | Confirmation w/ unknown address            |                 | Added   |
+// | Unknown    | Incomplete | Packet queued to unknown address           | Send probe      | Added   |
+// | Unknown    | Stale      | Probe w/ unknown address                   |                 | Added   |
+// | Incomplete | Incomplete | Retransmit timer expired                   | Send probe      | Changed |
+// | Incomplete | Reachable  | Solicited confirmation                     | Notify wakers   | Changed |
+// | Incomplete | Stale      | Unsolicited confirmation                   | Notify wakers   | Changed |
+// | Incomplete | Failed     | Max probes sent without reply              | Notify wakers   | Removed |
+// | Reachable  | Reachable  | Confirmation w/ different isRouter flag    | Update IsRouter |         |
+// | Reachable  | Stale      | Reachable timer expired                    |                 | Changed |
+// | Reachable  | Stale      | Probe or confirmation w/ different address |                 | Changed |
+// | Stale      | Reachable  | Solicited override confirmation            | Update LinkAddr | Changed |
+// | Stale      | Stale      | Override confirmation                      | Update LinkAddr | Changed |
+// | Stale      | Stale      | Probe w/ different address                 | Update LinkAddr | Changed |
+// | Stale      | Delay      | Packet sent                                |                 | Changed |
+// | Delay      | Reachable  | Upper-layer confirmation                   |                 | Changed |
+// | Delay      | Reachable  | Solicited override confirmation            | Update LinkAddr | Changed |
+// | Delay      | Stale      | Probe or confirmation w/ different address |                 | Changed |
+// | Delay      | Probe      | Delay timer expired                        | Send probe      | Changed |
+// | Probe      | Reachable  | Solicited override confirmation            | Update LinkAddr | Changed |
+// | Probe      | Reachable  | Solicited confirmation w/ same address     | Notify wakers   | Changed |
+// | Probe      | Stale      | Probe or confirmation w/ different address |                 | Changed |
+// | Probe      | Probe      | Retransmit timer expired                   | Send probe      | Changed |
+// | Probe      | Failed     | Max probes sent without reply              | Notify wakers   | Removed |
+// | Failed     |            | Unreachability timer expired               | Delete entry    |         |
+
+type testEntryEventType uint8
+
+const (
+	entryTestAdded testEntryEventType = iota
+	entryTestChanged
+	entryTestRemoved
+)
+
+func (t testEntryEventType) String() string {
+	switch t {
+	case entryTestAdded:
+		return "add"
+	case entryTestChanged:
+		return "change"
+	case entryTestRemoved:
+		return "remove"
+	default:
+		return fmt.Sprintf("unknown (%d)", t)
+	}
+}
+
+// Fields are exported for use with cmp.Diff.
+type testEntryEventInfo struct {
+	EventType testEntryEventType
+	NICID     tcpip.NICID
+	Addr      tcpip.Address
+	LinkAddr  tcpip.LinkAddress
+	State     NeighborState
+	UpdatedAt time.Time
+}
+
+func (e testEntryEventInfo) String() string {
+	return fmt.Sprintf("%s event for NIC #%d, addr=%q, linkAddr=%q, state=%q", e.EventType, e.NICID, e.Addr, e.LinkAddr, e.State)
+}
+
+// testNUDDispatcher implements NUDDispatcher to validate the dispatching of
+// events upon certain NUD state machine events.
+type testNUDDispatcher struct {
+	mu     sync.Mutex
+	events []testEntryEventInfo
+}
+
+var _ NUDDispatcher = (*testNUDDispatcher)(nil)
+
+func (d *testNUDDispatcher) queueEvent(e testEntryEventInfo) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	d.events = append(d.events, e)
+}
+
+func (d *testNUDDispatcher) OnNeighborAdded(nicID tcpip.NICID, addr tcpip.Address, linkAddr tcpip.LinkAddress, state NeighborState, updatedAt time.Time) {
+	d.queueEvent(testEntryEventInfo{
+		EventType: entryTestAdded,
+		NICID:     nicID,
+		Addr:      addr,
+		LinkAddr:  linkAddr,
+		State:     state,
+		UpdatedAt: updatedAt,
+	})
+}
+
+func (d *testNUDDispatcher) OnNeighborChanged(nicID tcpip.NICID, addr tcpip.Address, linkAddr tcpip.LinkAddress, state NeighborState, updatedAt time.Time) {
+	d.queueEvent(testEntryEventInfo{
+		EventType: entryTestChanged,
+		NICID:     nicID,
+		Addr:      addr,
+		LinkAddr:  linkAddr,
+		State:     state,
+		UpdatedAt: updatedAt,
+	})
+}
+
+func (d *testNUDDispatcher) OnNeighborRemoved(nicID tcpip.NICID, addr tcpip.Address, linkAddr tcpip.LinkAddress, state NeighborState, updatedAt time.Time) {
+	d.queueEvent(testEntryEventInfo{
+		EventType: entryTestRemoved,
+		NICID:     nicID,
+		Addr:      addr,
+		LinkAddr:  linkAddr,
+		State:     state,
+		UpdatedAt: updatedAt,
+	})
+}
+
+type entryTestLinkResolver struct {
+	mu     sync.Mutex
+	probes []entryTestProbeInfo
+}
+
+var _ LinkAddressResolver = (*entryTestLinkResolver)(nil)
+
+type entryTestProbeInfo struct {
+	RemoteAddress     tcpip.Address
+	RemoteLinkAddress tcpip.LinkAddress
+	LocalAddress      tcpip.Address
+}
+
+func (p entryTestProbeInfo) String() string {
+	return fmt.Sprintf("probe with RemoteAddress=%q, RemoteLinkAddress=%q, LocalAddress=%q", p.RemoteAddress, p.RemoteLinkAddress, p.LocalAddress)
+}
+
+// LinkAddressRequest sends a request for the LinkAddress of addr. Broadcasts
+// to the local network if linkAddr is the zero value.
+func (r *entryTestLinkResolver) LinkAddressRequest(addr, localAddr tcpip.Address, linkAddr tcpip.LinkAddress, linkEP LinkEndpoint) *tcpip.Error {
+	p := entryTestProbeInfo{
+		RemoteAddress:     addr,
+		RemoteLinkAddress: linkAddr,
+		LocalAddress:      localAddr,
+	}
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.probes = append(r.probes, p)
+	return nil
+}
+
+// ResolveStaticAddress attempts to resolve address without sending requests.
+// It either resolves the name immediately or returns the empty LinkAddress.
+func (r *entryTestLinkResolver) ResolveStaticAddress(addr tcpip.Address) (tcpip.LinkAddress, bool) {
+	return "", false
+}
+
+// LinkAddressProtocol returns the network protocol of the addresses this
+// resolver can resolve.
+func (r *entryTestLinkResolver) LinkAddressProtocol() tcpip.NetworkProtocolNumber {
+	return entryTestNetNumber
+}
+
+func entryTestSetup(c NUDConfigurations) (*neighborEntry, *testNUDDispatcher, *entryTestLinkResolver, *fakeClock) {
+	clock := newFakeClock()
+	disp := testNUDDispatcher{}
+	nic := NIC{
+		id:     entryTestNICID,
+		linkEP: nil, // entryTestLinkResolver doesn't use a LinkEndpoint
+		stack: &Stack{
+			clock:   clock,
+			nudDisp: &disp,
+		},
+	}
+
+	rng := rand.New(rand.NewSource(time.Now().UnixNano()))
+	nudState := NewNUDState(c, rng)
+	linkRes := entryTestLinkResolver{}
+	entry := newNeighborEntry(&nic, entryTestAddr1, entryTestAddr2, nudState, &linkRes)
+
+	// Stub out ndpState to verify modification of default routers.
+	nic.mu.ndp = ndpState{
+		nic:            &nic,
+		defaultRouters: make(map[tcpip.Address]defaultRouterState),
+	}
+
+	// Stub out the neighbor cache to verify deletion from the cache.
+	nic.neigh = &neighborCache{
+		nic:   &nic,
+		state: nudState,
+		cache: make(map[tcpip.Address]*neighborEntry, neighborCacheSize),
+	}
+	nic.neigh.cache[entryTestAddr1] = entry
+
+	return entry, &disp, &linkRes, clock
+}
+
+// TestEntryInitiallyUnknown verifies that the state of a newly created
+// neighborEntry is Unknown.
+func TestEntryInitiallyUnknown(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Unknown; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	clock.advance(c.RetransmitTimer)
+
+	// No probes should have been sent.
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, []entryTestProbeInfo(nil))
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	// No events should have been dispatched.
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, []testEntryEventInfo(nil)); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryUnknownToUnknownWhenConfirmationWithUnknownAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Unknown; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	clock.advance(time.Hour)
+
+	// No probes should have been sent.
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, []entryTestProbeInfo(nil))
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	// No events should have been dispatched.
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, []testEntryEventInfo(nil)); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryUnknownToIncomplete(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Incomplete; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+	}
+	{
+		nudDisp.mu.Lock()
+		diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...)
+		nudDisp.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+		}
+	}
+}
+
+func TestEntryUnknownToStale(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handleProbeLocked(entryTestLinkAddr1)
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	// No probes should have been sent.
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, []entryTestProbeInfo(nil))
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryIncompleteToIncompleteDoesNotChangeUpdatedAt(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	c.MaxMulticastProbes = 3
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Incomplete; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	updatedAt := e.neigh.UpdatedAt
+	e.mu.Unlock()
+
+	clock.advance(c.RetransmitTimer)
+
+	// UpdatedAt should remain the same during address resolution.
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.probes = nil
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	e.mu.Lock()
+	if got, want := e.neigh.UpdatedAt, updatedAt; got != want {
+		t.Errorf("got e.neigh.UpdatedAt = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	clock.advance(c.RetransmitTimer)
+
+	// UpdatedAt should change after failing address resolution. Timing out after
+	// sending the last probe transitions the entry to Failed.
+	{
+		wantProbes := []entryTestProbeInfo{
+			{
+				RemoteAddress:     entryTestAddr1,
+				RemoteLinkAddress: tcpip.LinkAddress(""),
+				LocalAddress:      entryTestAddr2,
+			},
+		}
+		linkRes.mu.Lock()
+		diff := cmp.Diff(linkRes.probes, wantProbes)
+		linkRes.mu.Unlock()
+		if diff != "" {
+			t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+		}
+	}
+
+	clock.advance(c.RetransmitTimer)
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestRemoved,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, notWant := e.neigh.UpdatedAt, updatedAt; got == notWant {
+		t.Errorf("expected e.neigh.UpdatedAt to change, got = %q", got)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryIncompleteToReachable(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Incomplete; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+// TestEntryAddsAndClearsWakers verifies that wakers are added when
+// addWakerLocked is called and cleared when address resolution finishes. In
+// this case, address resolution will finish when transitioning from Incomplete
+// to Reachable.
+func TestEntryAddsAndClearsWakers(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	w := sleep.Waker{}
+	s := sleep.Sleeper{}
+	s.AddWaker(&w, 123)
+	defer s.Done()
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	if got := e.wakers; got != nil {
+		t.Errorf("got e.wakers = %v, want = nil", got)
+	}
+	e.addWakerLocked(&w)
+	if got, want := w.IsAsserted(), false; got != want {
+		t.Errorf("waker.IsAsserted() = %t, want = %t", got, want)
+	}
+	if e.wakers == nil {
+		t.Error("expected e.wakers to be non-nil")
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if e.wakers != nil {
+		t.Errorf("got e.wakers = %v, want = nil", e.wakers)
+	}
+	if got, want := w.IsAsserted(), true; got != want {
+		t.Errorf("waker.IsAsserted() = %t, want = %t", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryIncompleteToReachableWithRouterFlag(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Incomplete; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  true,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.isRouter, true; got != want {
+		t.Errorf("got e.isRouter = %t, want = %t", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	if diff := cmp.Diff(linkRes.probes, wantProbes); diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+	linkRes.mu.Unlock()
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryIncompleteToStale(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Incomplete; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryIncompleteToFailed(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	c.MaxMulticastProbes = 3
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Incomplete; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	waitFor := c.RetransmitTimer * time.Duration(c.MaxMulticastProbes)
+	clock.advance(waitFor)
+
+	wantProbes := []entryTestProbeInfo{
+		// The Incomplete-to-Incomplete state transition is tested here by
+		// verifying that 3 reachability probes were sent.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestRemoved,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Failed; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+type testLocker struct{}
+
+var _ sync.Locker = (*testLocker)(nil)
+
+func (*testLocker) Lock()   {}
+func (*testLocker) Unlock() {}
+
+func TestEntryStaysReachableWhenConfirmationWithRouterFlag(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  true,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.isRouter, true; got != want {
+		t.Errorf("got e.isRouter = %t, want = %t", got, want)
+	}
+	e.nic.mu.ndp.defaultRouters[entryTestAddr1] = defaultRouterState{
+		invalidationJob: e.nic.stack.newJob(&testLocker{}, func() {}),
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.isRouter, false; got != want {
+		t.Errorf("got e.isRouter = %t, want = %t", got, want)
+	}
+	if _, ok := e.nic.mu.ndp.defaultRouters[entryTestAddr1]; ok {
+		t.Errorf("unexpected defaultRouter for %s", entryTestAddr1)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryStaysReachableWhenProbeWithSameAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleProbeLocked(entryTestLinkAddr1)
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.neigh.LinkAddr, entryTestLinkAddr1; got != want {
+		t.Errorf("got e.neigh.LinkAddr = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryReachableToStaleWhenTimeout(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	// Eliminate random factors from ReachableTime computation so the transition
+	// from Stale to Reachable will only take BaseReachableTime duration.
+	c.MinRandomFactor = 1
+	c.MaxRandomFactor = 1
+
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	clock.advance(c.BaseReachableTime)
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryReachableToStaleWhenProbeWithDifferentAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleProbeLocked(entryTestLinkAddr2)
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryReachableToStaleWhenConfirmationWithDifferentAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr2, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryReachableToStaleWhenConfirmationWithDifferentAddressAndOverride(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr2, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  true,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryStaysStaleWhenProbeWithSameAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleProbeLocked(entryTestLinkAddr1)
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.neigh.LinkAddr, entryTestLinkAddr1; got != want {
+		t.Errorf("got e.neigh.LinkAddr = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryStaleToReachableWhenSolicitedOverrideConfirmation(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr2, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  true,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.neigh.LinkAddr, entryTestLinkAddr2; got != want {
+		t.Errorf("got e.neigh.LinkAddr = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Reachable,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryStaleToStaleWhenOverrideConfirmation(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr2, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  true,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.neigh.LinkAddr, entryTestLinkAddr2; got != want {
+		t.Errorf("got e.neigh.LinkAddr = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryStaleToStaleWhenProbeUpdateAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleProbeLocked(entryTestLinkAddr2)
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.neigh.LinkAddr, entryTestLinkAddr2; got != want {
+		t.Errorf("got e.neigh.LinkAddr = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryStaleToDelay(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Delay; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryDelayToReachableWhenUpperLevelConfirmation(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	// Eliminate random factors from ReachableTime computation so the transition
+	// from Stale to Reachable will only take BaseReachableTime duration.
+	c.MinRandomFactor = 1
+	c.MaxRandomFactor = 1
+
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Delay; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleUpperLevelConfirmationLocked()
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	clock.advance(c.BaseReachableTime)
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryDelayToReachableWhenSolicitedOverrideConfirmation(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	c.MaxMulticastProbes = 1
+	// Eliminate random factors from ReachableTime computation so the transition
+	// from Stale to Reachable will only take BaseReachableTime duration.
+	c.MinRandomFactor = 1
+	c.MaxRandomFactor = 1
+
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Delay; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr2, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  true,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.neigh.LinkAddr, entryTestLinkAddr2; got != want {
+		t.Errorf("got e.neigh.LinkAddr = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	clock.advance(c.BaseReachableTime)
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Reachable,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryStaysDelayWhenOverrideConfirmationWithSameAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Delay; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  true,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Delay; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.neigh.LinkAddr, entryTestLinkAddr1; got != want {
+		t.Errorf("got e.neigh.LinkAddr = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryDelayToStaleWhenProbeWithDifferentAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Delay; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleProbeLocked(entryTestLinkAddr2)
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryDelayToStaleWhenConfirmationWithDifferentAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, _ := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Delay; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr2, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  true,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantProbes := []entryTestProbeInfo{
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryDelayToProbe(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	if got, want := e.neigh.State, Delay; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	clock.advance(c.DelayFirstProbeTime)
+
+	wantProbes := []entryTestProbeInfo{
+		// The first probe is caused by the Unknown-to-Incomplete transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		// The second probe is caused by the Delay-to-Probe transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Probe; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryProbeToStaleWhenProbeWithDifferentAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	e.mu.Unlock()
+
+	clock.advance(c.DelayFirstProbeTime)
+
+	wantProbes := []entryTestProbeInfo{
+		// The first probe is caused by the Unknown-to-Incomplete transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		// The second probe is caused by the Delay-to-Probe transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Probe; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleProbeLocked(entryTestLinkAddr2)
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryProbeToStaleWhenConfirmationWithDifferentAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	e.mu.Unlock()
+
+	clock.advance(c.DelayFirstProbeTime)
+
+	wantProbes := []entryTestProbeInfo{
+		// The first probe is caused by the Unknown-to-Incomplete transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		// The second probe is caused by the Delay-to-Probe transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Probe; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr2, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  true,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Stale; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryStaysProbeWhenOverrideConfirmationWithSameAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	e.mu.Unlock()
+
+	clock.advance(c.DelayFirstProbeTime)
+
+	wantProbes := []entryTestProbeInfo{
+		// The first probe is caused by the Unknown-to-Incomplete transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		// The second probe is caused by the Delay-to-Probe transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Probe; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  true,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Probe; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.neigh.LinkAddr, entryTestLinkAddr1; got != want {
+		t.Errorf("got e.neigh.LinkAddr = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryProbeToReachableWhenSolicitedOverrideConfirmation(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	// Eliminate random factors from ReachableTime computation so the transition
+	// from Stale to Reachable will only take BaseReachableTime duration.
+	c.MinRandomFactor = 1
+	c.MaxRandomFactor = 1
+
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	e.mu.Unlock()
+
+	clock.advance(c.DelayFirstProbeTime)
+
+	wantProbes := []entryTestProbeInfo{
+		// The first probe is caused by the Unknown-to-Incomplete transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		// The second probe is caused by the Delay-to-Probe transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Probe; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr2, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  true,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	if got, want := e.neigh.LinkAddr, entryTestLinkAddr2; got != want {
+		t.Errorf("got e.neigh.LinkAddr = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	clock.advance(c.BaseReachableTime)
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Reachable,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr2,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryProbeToReachableWhenSolicitedConfirmationWithSameAddress(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	// Eliminate random factors from ReachableTime computation so the transition
+	// from Stale to Reachable will only take BaseReachableTime duration.
+	c.MinRandomFactor = 1
+	c.MaxRandomFactor = 1
+
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	e.mu.Unlock()
+
+	clock.advance(c.DelayFirstProbeTime)
+
+	wantProbes := []entryTestProbeInfo{
+		// The first probe is caused by the Unknown-to-Incomplete transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		// The second probe is caused by the Delay-to-Probe transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Probe; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: true,
+		Override:  false,
+		IsRouter:  false,
+	})
+	if got, want := e.neigh.State, Reachable; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+
+	clock.advance(c.BaseReachableTime)
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Reachable,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+}
+
+func TestEntryProbeToFailed(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	c.MaxMulticastProbes = 3
+	c.MaxUnicastProbes = 3
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	e.mu.Unlock()
+
+	waitFor := c.DelayFirstProbeTime + c.RetransmitTimer*time.Duration(c.MaxUnicastProbes)
+	clock.advance(waitFor)
+
+	wantProbes := []entryTestProbeInfo{
+		// The first probe is caused by the Unknown-to-Incomplete transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		// The next three probe are caused by the Delay-to-Probe transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+		{
+			EventType: entryTestRemoved,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	e.mu.Lock()
+	if got, want := e.neigh.State, Failed; got != want {
+		t.Errorf("got e.neigh.State = %q, want = %q", got, want)
+	}
+	e.mu.Unlock()
+}
+
+func TestEntryFailedGetsDeleted(t *testing.T) {
+	c := DefaultNUDConfigurations()
+	c.MaxMulticastProbes = 3
+	c.MaxUnicastProbes = 3
+	e, nudDisp, linkRes, clock := entryTestSetup(c)
+
+	// Verify the cache contains the entry.
+	if _, ok := e.nic.neigh.cache[entryTestAddr1]; !ok {
+		t.Errorf("expected entry %q to exist in the neighbor cache", entryTestAddr1)
+	}
+
+	e.mu.Lock()
+	e.handlePacketQueuedLocked()
+	e.handleConfirmationLocked(entryTestLinkAddr1, ReachabilityConfirmationFlags{
+		Solicited: false,
+		Override:  false,
+		IsRouter:  false,
+	})
+	e.handlePacketQueuedLocked()
+	e.mu.Unlock()
+
+	waitFor := c.DelayFirstProbeTime + c.RetransmitTimer*time.Duration(c.MaxUnicastProbes) + c.UnreachableTime
+	clock.advance(waitFor)
+
+	wantProbes := []entryTestProbeInfo{
+		// The first probe is caused by the Unknown-to-Incomplete transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: tcpip.LinkAddress(""),
+			LocalAddress:      entryTestAddr2,
+		},
+		// The next three probe are caused by the Delay-to-Probe transition.
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+		{
+			RemoteAddress:     entryTestAddr1,
+			RemoteLinkAddress: entryTestLinkAddr1,
+			LocalAddress:      entryTestAddr2,
+		},
+	}
+	linkRes.mu.Lock()
+	diff := cmp.Diff(linkRes.probes, wantProbes)
+	linkRes.mu.Unlock()
+	if diff != "" {
+		t.Fatalf("link address resolver probes mismatch (-got, +want):\n%s", diff)
+	}
+
+	wantEvents := []testEntryEventInfo{
+		{
+			EventType: entryTestAdded,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  tcpip.LinkAddress(""),
+			State:     Incomplete,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Stale,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Delay,
+		},
+		{
+			EventType: entryTestChanged,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+		{
+			EventType: entryTestRemoved,
+			NICID:     entryTestNICID,
+			Addr:      entryTestAddr1,
+			LinkAddr:  entryTestLinkAddr1,
+			State:     Probe,
+		},
+	}
+	nudDisp.mu.Lock()
+	if diff := cmp.Diff(nudDisp.events, wantEvents, eventDiffOpts()...); diff != "" {
+		t.Errorf("nud dispatcher events mismatch (-got, +want):\n%s", diff)
+	}
+	nudDisp.mu.Unlock()
+
+	// Verify the cache no longer contains the entry.
+	if _, ok := e.nic.neigh.cache[entryTestAddr1]; ok {
+		t.Errorf("entry %q should have been deleted from the neighbor cache", entryTestAddr1)
+	}
+}
diff --git a/pkg/tcpip/stack/neighborstate_string.go b/pkg/tcpip/stack/neighborstate_string.go
new file mode 100644
index 000000000..aa7311ec6
--- /dev/null
+++ b/pkg/tcpip/stack/neighborstate_string.go
@@ -0,0 +1,44 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Code generated by "stringer -type NeighborState"; DO NOT EDIT.
+
+package stack
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[Unknown-0]
+	_ = x[Incomplete-1]
+	_ = x[Reachable-2]
+	_ = x[Stale-3]
+	_ = x[Delay-4]
+	_ = x[Probe-5]
+	_ = x[Static-6]
+	_ = x[Failed-7]
+}
+
+const _NeighborState_name = "UnknownIncompleteReachableStaleDelayProbeStaticFailed"
+
+var _NeighborState_index = [...]uint8{0, 7, 17, 26, 31, 36, 41, 47, 53}
+
+func (i NeighborState) String() string {
+	if i >= NeighborState(len(_NeighborState_index)-1) {
+		return "NeighborState(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _NeighborState_name[_NeighborState_index[i]:_NeighborState_index[i+1]]
+}
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 9256d4d43..f21066fce 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -16,6 +16,7 @@ package stack
 
 import (
 	"fmt"
+	"math/rand"
 	"reflect"
 	"sort"
 	"strings"
@@ -45,6 +46,7 @@ type NIC struct {
 	context NICContext
 
 	stats NICStats
+	neigh *neighborCache
 
 	mu struct {
 		sync.RWMutex
@@ -141,6 +143,16 @@ func newNIC(stack *Stack, id tcpip.NICID, name string, ep LinkEndpoint, ctx NICC
 		nic.mu.packetEPs[netProto.Number()] = []PacketEndpoint{}
 	}
 
+	// Check for Neighbor Unreachability Detection support.
+	if ep.Capabilities()&CapabilityResolutionRequired != 0 && len(stack.linkAddrResolvers) != 0 {
+		rng := rand.New(rand.NewSource(stack.clock.NowNanoseconds()))
+		nic.neigh = &neighborCache{
+			nic:   nic,
+			state: NewNUDState(stack.nudConfigs, rng),
+			cache: make(map[tcpip.Address]*neighborEntry, neighborCacheSize),
+		}
+	}
+
 	nic.linkEP.Attach(nic)
 
 	return nic
@@ -1540,6 +1552,27 @@ func (n *NIC) setNDPConfigs(c NDPConfigurations) {
 	n.mu.Unlock()
 }
 
+// NUDConfigs gets the NUD configurations for n.
+func (n *NIC) NUDConfigs() (NUDConfigurations, *tcpip.Error) {
+	if n.neigh == nil {
+		return NUDConfigurations{}, tcpip.ErrNotSupported
+	}
+	return n.neigh.config(), nil
+}
+
+// setNUDConfigs sets the NUD configurations for n.
+//
+// Note, if c contains invalid NUD configuration values, it will be fixed to
+// use default values for the erroneous values.
+func (n *NIC) setNUDConfigs(c NUDConfigurations) *tcpip.Error {
+	if n.neigh == nil {
+		return tcpip.ErrNotSupported
+	}
+	c.resetInvalidFields()
+	n.neigh.setConfig(c)
+	return nil
+}
+
 // handleNDPRA handles an NDP Router Advertisement message that arrived on n.
 func (n *NIC) handleNDPRA(ip tcpip.Address, ra header.NDPRouterAdvert) {
 	n.mu.Lock()
diff --git a/pkg/tcpip/stack/nud.go b/pkg/tcpip/stack/nud.go
new file mode 100644
index 000000000..f848d50ad
--- /dev/null
+++ b/pkg/tcpip/stack/nud.go
@@ -0,0 +1,466 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"math"
+	"sync"
+	"time"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+)
+
+const (
+	// defaultBaseReachableTime is the default base duration for computing the
+	// random reachable time.
+	//
+	// Reachable time is the duration for which a neighbor is considered
+	// reachable after a positive reachability confirmation is received. It is a
+	// function of a uniformly distributed random value between the minimum and
+	// maximum random factors, multiplied by the base reachable time. Using a
+	// random component eliminates the possibility that Neighbor Unreachability
+	// Detection messages will synchronize with each other.
+	//
+	// Default taken from REACHABLE_TIME of RFC 4861 section 10.
+	defaultBaseReachableTime = 30 * time.Second
+
+	// minimumBaseReachableTime is the minimum base duration for computing the
+	// random reachable time.
+	//
+	// Minimum = 1ms
+	minimumBaseReachableTime = time.Millisecond
+
+	// defaultMinRandomFactor is the default minimum value of the random factor
+	// used for computing reachable time.
+	//
+	// Default taken from MIN_RANDOM_FACTOR of RFC 4861 section 10.
+	defaultMinRandomFactor = 0.5
+
+	// defaultMaxRandomFactor is the default maximum value of the random factor
+	// used for computing reachable time.
+	//
+	// The default value depends on the value of MinRandomFactor.
+	// If MinRandomFactor is less than MAX_RANDOM_FACTOR of RFC 4861 section 10,
+	// the value from the RFC will be used; otherwise, the default is
+	// MinRandomFactor multiplied by three.
+	defaultMaxRandomFactor = 1.5
+
+	// defaultRetransmitTimer is the default amount of time to wait between
+	// sending reachability probes.
+	//
+	// Default taken from RETRANS_TIMER of RFC 4861 section 10.
+	defaultRetransmitTimer = time.Second
+
+	// minimumRetransmitTimer is the minimum amount of time to wait between
+	// sending reachability probes.
+	//
+	// Note, RFC 4861 does not impose a minimum Retransmit Timer, but we do here
+	// to make sure the messages are not sent all at once. We also come to this
+	// value because in the RetransmitTimer field of a Router Advertisement, a
+	// value of 0 means unspecified, so the smallest valid value is 1. Note, the
+	// unit of the RetransmitTimer field in the Router Advertisement is
+	// milliseconds.
+	minimumRetransmitTimer = time.Millisecond
+
+	// defaultDelayFirstProbeTime is the default duration to wait for a
+	// non-Neighbor-Discovery related protocol to reconfirm reachability after
+	// entering the DELAY state. After this time, a reachability probe will be
+	// sent and the entry will transition to the PROBE state.
+	//
+	// Default taken from DELAY_FIRST_PROBE_TIME of RFC 4861 section 10.
+	defaultDelayFirstProbeTime = 5 * time.Second
+
+	// defaultMaxMulticastProbes is the default number of reachabililty probes
+	// to send before concluding negative reachability and deleting the neighbor
+	// entry from the INCOMPLETE state.
+	//
+	// Default taken from MAX_MULTICAST_SOLICIT of RFC 4861 section 10.
+	defaultMaxMulticastProbes = 3
+
+	// defaultMaxUnicastProbes is the default number of reachability probes to
+	// send before concluding retransmission from within the PROBE state should
+	// cease and the entry SHOULD be deleted.
+	//
+	// Default taken from MAX_UNICASE_SOLICIT of RFC 4861 section 10.
+	defaultMaxUnicastProbes = 3
+
+	// defaultMaxAnycastDelayTime is the default time in which the stack SHOULD
+	// delay sending a response for a random time between 0 and this time, if the
+	// target address is an anycast address.
+	//
+	// Default taken from MAX_ANYCAST_DELAY_TIME of RFC 4861 section 10.
+	defaultMaxAnycastDelayTime = time.Second
+
+	// defaultMaxReachbilityConfirmations is the default amount of unsolicited
+	// reachability confirmation messages a node MAY send to all-node multicast
+	// address when it determines its link-layer address has changed.
+	//
+	// Default taken from MAX_NEIGHBOR_ADVERTISEMENT of RFC 4861 section 10.
+	defaultMaxReachbilityConfirmations = 3
+
+	// defaultUnreachableTime is the default duration for how long an entry will
+	// remain in the FAILED state before being removed from the neighbor cache.
+	//
+	// Note, there is no equivalent protocol constant defined in RFC 4861. It
+	// leaves the specifics of any garbage collection mechanism up to the
+	// implementation.
+	defaultUnreachableTime = 5 * time.Second
+)
+
+// NUDDispatcher is the interface integrators of netstack must implement to
+// receive and handle NUD related events.
+type NUDDispatcher interface {
+	// OnNeighborAdded will be called when a new entry is added to a NIC's (with
+	// ID nicID) neighbor table.
+	//
+	// This function is permitted to block indefinitely without interfering with
+	// the stack's operation.
+	//
+	// May be called concurrently.
+	OnNeighborAdded(nicID tcpip.NICID, ipAddr tcpip.Address, linkAddr tcpip.LinkAddress, state NeighborState, updatedAt time.Time)
+
+	// OnNeighborChanged will be called when an entry in a NIC's (with ID nicID)
+	// neighbor table changes state and/or link address.
+	//
+	// This function is permitted to block indefinitely without interfering with
+	// the stack's operation.
+	//
+	// May be called concurrently.
+	OnNeighborChanged(nicID tcpip.NICID, ipAddr tcpip.Address, linkAddr tcpip.LinkAddress, state NeighborState, updatedAt time.Time)
+
+	// OnNeighborRemoved will be called when an entry is removed from a NIC's
+	// (with ID nicID) neighbor table.
+	//
+	// This function is permitted to block indefinitely without interfering with
+	// the stack's operation.
+	//
+	// May be called concurrently.
+	OnNeighborRemoved(nicID tcpip.NICID, ipAddr tcpip.Address, linkAddr tcpip.LinkAddress, state NeighborState, updatedAt time.Time)
+}
+
+// ReachabilityConfirmationFlags describes the flags used within a reachability
+// confirmation (e.g. ARP reply or Neighbor Advertisement for ARP or NDP,
+// respectively).
+type ReachabilityConfirmationFlags struct {
+	// Solicited indicates that the advertisement was sent in response to a
+	// reachability probe.
+	Solicited bool
+
+	// Override indicates that the reachability confirmation should override an
+	// existing neighbor cache entry and update the cached link-layer address.
+	// When Override is not set the confirmation will not update a cached
+	// link-layer address, but will update an existing neighbor cache entry for
+	// which no link-layer address is known.
+	Override bool
+
+	// IsRouter indicates that the sender is a router.
+	IsRouter bool
+}
+
+// NUDHandler communicates external events to the Neighbor Unreachability
+// Detection state machine, which is implemented per-interface. This is used by
+// network endpoints to inform the Neighbor Cache of probes and confirmations.
+type NUDHandler interface {
+	// HandleProbe processes an incoming neighbor probe (e.g. ARP request or
+	// Neighbor Solicitation for ARP or NDP, respectively). Validation of the
+	// probe needs to be performed before calling this function since the
+	// Neighbor Cache doesn't have access to view the NIC's assigned addresses.
+	HandleProbe(remoteAddr, localAddr tcpip.Address, protocol tcpip.NetworkProtocolNumber, remoteLinkAddr tcpip.LinkAddress)
+
+	// HandleConfirmation processes an incoming neighbor confirmation (e.g. ARP
+	// reply or Neighbor Advertisement for ARP or NDP, respectively).
+	HandleConfirmation(addr tcpip.Address, linkAddr tcpip.LinkAddress, flags ReachabilityConfirmationFlags)
+
+	// HandleUpperLevelConfirmation processes an incoming upper-level protocol
+	// (e.g. TCP acknowledgements) reachability confirmation.
+	HandleUpperLevelConfirmation(addr tcpip.Address)
+}
+
+// NUDConfigurations is the NUD configurations for the netstack. This is used
+// by the neighbor cache to operate the NUD state machine on each device in the
+// local network.
+type NUDConfigurations struct {
+	// BaseReachableTime is the base duration for computing the random reachable
+	// time.
+	//
+	// Reachable time is the duration for which a neighbor is considered
+	// reachable after a positive reachability confirmation is received. It is a
+	// function of uniformly distributed random value between minRandomFactor and
+	// maxRandomFactor multiplied by baseReachableTime. Using a random component
+	// eliminates the possibility that Neighbor Unreachability Detection messages
+	// will synchronize with each other.
+	//
+	// After this time, a neighbor entry will transition from REACHABLE to STALE
+	// state.
+	//
+	// Must be greater than 0.
+	BaseReachableTime time.Duration
+
+	// LearnBaseReachableTime enables learning BaseReachableTime during runtime
+	// from the neighbor discovery protocol, if supported.
+	//
+	// TODO(gvisor.dev/issue/2240): Implement this NUD configuration option.
+	LearnBaseReachableTime bool
+
+	// MinRandomFactor is the minimum value of the random factor used for
+	// computing reachable time.
+	//
+	// See BaseReachbleTime for more information on computing the reachable time.
+	//
+	// Must be greater than 0.
+	MinRandomFactor float32
+
+	// MaxRandomFactor is the maximum value of the random factor used for
+	// computing reachabile time.
+	//
+	// See BaseReachbleTime for more information on computing the reachable time.
+	//
+	// Must be great than or equal to MinRandomFactor.
+	MaxRandomFactor float32
+
+	// RetransmitTimer is the duration between retransmission of reachability
+	// probes in the PROBE state.
+	RetransmitTimer time.Duration
+
+	// LearnRetransmitTimer enables learning RetransmitTimer during runtime from
+	// the neighbor discovery protocol, if supported.
+	//
+	// TODO(gvisor.dev/issue/2241): Implement this NUD configuration option.
+	LearnRetransmitTimer bool
+
+	// DelayFirstProbeTime is the duration to wait for a non-Neighbor-Discovery
+	// related protocol to reconfirm reachability after entering the DELAY state.
+	// After this time, a reachability probe will be sent and the entry will
+	// transition to the PROBE state.
+	//
+	// Must be greater than 0.
+	DelayFirstProbeTime time.Duration
+
+	// MaxMulticastProbes is the number of reachability probes to send before
+	// concluding negative reachability and deleting the neighbor entry from the
+	// INCOMPLETE state.
+	//
+	// Must be greater than 0.
+	MaxMulticastProbes uint32
+
+	// MaxUnicastProbes is the number of reachability probes to send before
+	// concluding retransmission from within the PROBE state should cease and
+	// entry SHOULD be deleted.
+	//
+	// Must be greater than 0.
+	MaxUnicastProbes uint32
+
+	// MaxAnycastDelayTime is the time in which the stack SHOULD delay sending a
+	// response for a random time between 0 and this time, if the target address
+	// is an anycast address.
+	//
+	// TODO(gvisor.dev/issue/2242): Use this option when sending solicited
+	// neighbor confirmations to anycast addresses and proxying neighbor
+	// confirmations.
+	MaxAnycastDelayTime time.Duration
+
+	// MaxReachabilityConfirmations is the number of unsolicited reachability
+	// confirmation messages a node MAY send to all-node multicast address when
+	// it determines its link-layer address has changed.
+	//
+	// TODO(gvisor.dev/issue/2246): Discuss if implementation of this NUD
+	// configuration option is necessary.
+	MaxReachabilityConfirmations uint32
+
+	// UnreachableTime describes how long an entry will remain in the FAILED
+	// state before being removed from the neighbor cache.
+	UnreachableTime time.Duration
+}
+
+// DefaultNUDConfigurations returns a NUDConfigurations populated with default
+// values defined by RFC 4861 section 10.
+func DefaultNUDConfigurations() NUDConfigurations {
+	return NUDConfigurations{
+		BaseReachableTime:            defaultBaseReachableTime,
+		LearnBaseReachableTime:       true,
+		MinRandomFactor:              defaultMinRandomFactor,
+		MaxRandomFactor:              defaultMaxRandomFactor,
+		RetransmitTimer:              defaultRetransmitTimer,
+		LearnRetransmitTimer:         true,
+		DelayFirstProbeTime:          defaultDelayFirstProbeTime,
+		MaxMulticastProbes:           defaultMaxMulticastProbes,
+		MaxUnicastProbes:             defaultMaxUnicastProbes,
+		MaxAnycastDelayTime:          defaultMaxAnycastDelayTime,
+		MaxReachabilityConfirmations: defaultMaxReachbilityConfirmations,
+		UnreachableTime:              defaultUnreachableTime,
+	}
+}
+
+// resetInvalidFields modifies an invalid NDPConfigurations with valid values.
+// If invalid values are present in c, the corresponding default values will be
+// used instead. This is needed to check, and conditionally fix, user-specified
+// NUDConfigurations.
+func (c *NUDConfigurations) resetInvalidFields() {
+	if c.BaseReachableTime < minimumBaseReachableTime {
+		c.BaseReachableTime = defaultBaseReachableTime
+	}
+	if c.MinRandomFactor <= 0 {
+		c.MinRandomFactor = defaultMinRandomFactor
+	}
+	if c.MaxRandomFactor < c.MinRandomFactor {
+		c.MaxRandomFactor = calcMaxRandomFactor(c.MinRandomFactor)
+	}
+	if c.RetransmitTimer < minimumRetransmitTimer {
+		c.RetransmitTimer = defaultRetransmitTimer
+	}
+	if c.DelayFirstProbeTime == 0 {
+		c.DelayFirstProbeTime = defaultDelayFirstProbeTime
+	}
+	if c.MaxMulticastProbes == 0 {
+		c.MaxMulticastProbes = defaultMaxMulticastProbes
+	}
+	if c.MaxUnicastProbes == 0 {
+		c.MaxUnicastProbes = defaultMaxUnicastProbes
+	}
+	if c.UnreachableTime == 0 {
+		c.UnreachableTime = defaultUnreachableTime
+	}
+}
+
+// calcMaxRandomFactor calculates the maximum value of the random factor used
+// for computing reachable time. This function is necessary for when the
+// default specified in RFC 4861 section 10 is less than the current
+// MinRandomFactor.
+//
+// Assumes minRandomFactor is positive since validation of the minimum value
+// should come before the validation of the maximum.
+func calcMaxRandomFactor(minRandomFactor float32) float32 {
+	if minRandomFactor > defaultMaxRandomFactor {
+		return minRandomFactor * 3
+	}
+	return defaultMaxRandomFactor
+}
+
+// A Rand is a source of random numbers.
+type Rand interface {
+	// Float32 returns, as a float32, a pseudo-random number in [0.0,1.0).
+	Float32() float32
+}
+
+// NUDState stores states needed for calculating reachable time.
+type NUDState struct {
+	rng Rand
+
+	// mu protects the fields below.
+	//
+	// It is necessary for NUDState to handle its own locking since neighbor
+	// entries may access the NUD state from within the goroutine spawned by
+	// time.AfterFunc(). This goroutine may run concurrently with the main
+	// process for controlling the neighbor cache and would otherwise introduce
+	// race conditions if NUDState was not locked properly.
+	mu sync.RWMutex
+
+	config NUDConfigurations
+
+	// reachableTime is the duration to wait for a REACHABLE entry to
+	// transition into STALE after inactivity. This value is calculated with
+	// the algorithm defined in RFC 4861 section 6.3.2.
+	reachableTime time.Duration
+
+	expiration            time.Time
+	prevBaseReachableTime time.Duration
+	prevMinRandomFactor   float32
+	prevMaxRandomFactor   float32
+}
+
+// NewNUDState returns new NUDState using c as configuration and the specified
+// random number generator for use in recomputing ReachableTime.
+func NewNUDState(c NUDConfigurations, rng Rand) *NUDState {
+	s := &NUDState{
+		rng: rng,
+	}
+	s.config = c
+	return s
+}
+
+// Config returns the NUD configuration.
+func (s *NUDState) Config() NUDConfigurations {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.config
+}
+
+// SetConfig replaces the existing NUD configurations with c.
+func (s *NUDState) SetConfig(c NUDConfigurations) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.config = c
+}
+
+// ReachableTime returns the duration to wait for a REACHABLE entry to
+// transition into STALE after inactivity. This value is recalculated for new
+// values of BaseReachableTime, MinRandomFactor, and MaxRandomFactor using the
+// algorithm defined in RFC 4861 section 6.3.2.
+func (s *NUDState) ReachableTime() time.Duration {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	if time.Now().After(s.expiration) ||
+		s.config.BaseReachableTime != s.prevBaseReachableTime ||
+		s.config.MinRandomFactor != s.prevMinRandomFactor ||
+		s.config.MaxRandomFactor != s.prevMaxRandomFactor {
+		return s.recomputeReachableTimeLocked()
+	}
+	return s.reachableTime
+}
+
+// recomputeReachableTimeLocked forces a recalculation of ReachableTime using
+// the algorithm defined in RFC 4861 section 6.3.2.
+//
+// This SHOULD automatically be invoked during certain situations, as per
+// RFC 4861 section 6.3.4:
+//
+//    If the received Reachable Time value is non-zero, the host SHOULD set its
+//    BaseReachableTime variable to the received value.  If the new value
+//    differs from the previous value, the host SHOULD re-compute a new random
+//    ReachableTime value.  ReachableTime is computed as a uniformly
+//    distributed random value between MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR
+//    times the BaseReachableTime.  Using a random component eliminates the
+//    possibility that Neighbor Unreachability Detection messages will
+//    synchronize with each other.
+//
+//    In most cases, the advertised Reachable Time value will be the same in
+//    consecutive Router Advertisements, and a host's BaseReachableTime rarely
+//    changes.  In such cases, an implementation SHOULD ensure that a new
+//    random value gets re-computed at least once every few hours.
+//
+// s.mu MUST be locked for writing.
+func (s *NUDState) recomputeReachableTimeLocked() time.Duration {
+	s.prevBaseReachableTime = s.config.BaseReachableTime
+	s.prevMinRandomFactor = s.config.MinRandomFactor
+	s.prevMaxRandomFactor = s.config.MaxRandomFactor
+
+	randomFactor := s.config.MinRandomFactor + s.rng.Float32()*(s.config.MaxRandomFactor-s.config.MinRandomFactor)
+
+	// Check for overflow, given that minRandomFactor and maxRandomFactor are
+	// guaranteed to be positive numbers.
+	if float32(math.MaxInt64)/randomFactor < float32(s.config.BaseReachableTime) {
+		s.reachableTime = time.Duration(math.MaxInt64)
+	} else if randomFactor == 1 {
+		// Avoid loss of precision when a large base reachable time is used.
+		s.reachableTime = s.config.BaseReachableTime
+	} else {
+		reachableTime := int64(float32(s.config.BaseReachableTime) * randomFactor)
+		s.reachableTime = time.Duration(reachableTime)
+	}
+
+	s.expiration = time.Now().Add(2 * time.Hour)
+	return s.reachableTime
+}
diff --git a/pkg/tcpip/stack/nud_test.go b/pkg/tcpip/stack/nud_test.go
new file mode 100644
index 000000000..2494ee610
--- /dev/null
+++ b/pkg/tcpip/stack/nud_test.go
@@ -0,0 +1,795 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack_test
+
+import (
+	"math"
+	"testing"
+	"time"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/link/channel"
+	"gvisor.dev/gvisor/pkg/tcpip/network/ipv6"
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+)
+
+const (
+	defaultBaseReachableTime           = 30 * time.Second
+	minimumBaseReachableTime           = time.Millisecond
+	defaultMinRandomFactor             = 0.5
+	defaultMaxRandomFactor             = 1.5
+	defaultRetransmitTimer             = time.Second
+	minimumRetransmitTimer             = time.Millisecond
+	defaultDelayFirstProbeTime         = 5 * time.Second
+	defaultMaxMulticastProbes          = 3
+	defaultMaxUnicastProbes            = 3
+	defaultMaxAnycastDelayTime         = time.Second
+	defaultMaxReachbilityConfirmations = 3
+	defaultUnreachableTime             = 5 * time.Second
+
+	defaultFakeRandomNum = 0.5
+)
+
+// fakeRand is a deterministic random number generator.
+type fakeRand struct {
+	num float32
+}
+
+var _ stack.Rand = (*fakeRand)(nil)
+
+func (f *fakeRand) Float32() float32 {
+	return f.num
+}
+
+// TestSetNUDConfigurationFailsForBadNICID tests to make sure we get an error if
+// we attempt to update NUD configurations using an invalid NICID.
+func TestSetNUDConfigurationFailsForBadNICID(t *testing.T) {
+	s := stack.New(stack.Options{
+		// A neighbor cache is required to store NUDConfigurations. The networking
+		// stack will only allocate neighbor caches if a protocol providing link
+		// address resolution is specified (e.g. ARP or IPv6).
+		NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+	})
+
+	// No NIC with ID 1 yet.
+	config := stack.NUDConfigurations{}
+	if err := s.SetNUDConfigurations(1, config); err != tcpip.ErrUnknownNICID {
+		t.Fatalf("got s.SetNDPConfigurations(1, %+v) = %v, want = %s", config, err, tcpip.ErrUnknownNICID)
+	}
+}
+
+// TestNUDConfigurationFailsForNotSupported tests to make sure we get a
+// NotSupported error if we attempt to retrieve NUD configurations when the
+// stack doesn't support NUD.
+//
+// The stack will report to not support NUD if a neighbor cache for a given NIC
+// is not allocated. The networking stack will only allocate neighbor caches if
+// a protocol providing link address resolution is specified (e.g. ARP, IPv6).
+func TestNUDConfigurationFailsForNotSupported(t *testing.T) {
+	const nicID = 1
+
+	e := channel.New(0, 1280, linkAddr1)
+	e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+	s := stack.New(stack.Options{
+		NUDConfigs: stack.DefaultNUDConfigurations(),
+	})
+	if err := s.CreateNIC(nicID, e); err != nil {
+		t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+	}
+	if _, err := s.NUDConfigurations(nicID); err != tcpip.ErrNotSupported {
+		t.Fatalf("got s.NDPConfigurations(%d) = %v, want = %s", nicID, err, tcpip.ErrNotSupported)
+	}
+}
+
+// TestNUDConfigurationFailsForNotSupported tests to make sure we get a
+// NotSupported error if we attempt to set NUD configurations when the stack
+// doesn't support NUD.
+//
+// The stack will report to not support NUD if a neighbor cache for a given NIC
+// is not allocated. The networking stack will only allocate neighbor caches if
+// a protocol providing link address resolution is specified (e.g. ARP, IPv6).
+func TestSetNUDConfigurationFailsForNotSupported(t *testing.T) {
+	const nicID = 1
+
+	e := channel.New(0, 1280, linkAddr1)
+	e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+	s := stack.New(stack.Options{
+		NUDConfigs: stack.DefaultNUDConfigurations(),
+	})
+	if err := s.CreateNIC(nicID, e); err != nil {
+		t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+	}
+
+	config := stack.NUDConfigurations{}
+	if err := s.SetNUDConfigurations(nicID, config); err != tcpip.ErrNotSupported {
+		t.Fatalf("got s.SetNDPConfigurations(%d, %+v) = %v, want = %s", nicID, config, err, tcpip.ErrNotSupported)
+	}
+}
+
+// TestDefaultNUDConfigurationIsValid verifies that calling
+// resetInvalidFields() on the result of DefaultNUDConfigurations() does not
+// change anything. DefaultNUDConfigurations() should return a valid
+// NUDConfigurations.
+func TestDefaultNUDConfigurations(t *testing.T) {
+	const nicID = 1
+
+	e := channel.New(0, 1280, linkAddr1)
+	e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+	s := stack.New(stack.Options{
+		// A neighbor cache is required to store NUDConfigurations. The networking
+		// stack will only allocate neighbor caches if a protocol providing link
+		// address resolution is specified (e.g. ARP or IPv6).
+		NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+		NUDConfigs:       stack.DefaultNUDConfigurations(),
+	})
+	if err := s.CreateNIC(nicID, e); err != nil {
+		t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+	}
+	c, err := s.NUDConfigurations(nicID)
+	if err != nil {
+		t.Fatalf("got stack.NUDConfigurations(%d) = %s", nicID, err)
+	}
+	if got, want := c, stack.DefaultNUDConfigurations(); got != want {
+		t.Errorf("got stack.NUDConfigurations(%d) = %+v, want = %+v", nicID, got, want)
+	}
+}
+
+func TestNUDConfigurationsBaseReachableTime(t *testing.T) {
+	tests := []struct {
+		name              string
+		baseReachableTime time.Duration
+		want              time.Duration
+	}{
+		// Invalid cases
+		{
+			name:              "EqualToZero",
+			baseReachableTime: 0,
+			want:              defaultBaseReachableTime,
+		},
+		// Valid cases
+		{
+			name:              "MoreThanZero",
+			baseReachableTime: time.Millisecond,
+			want:              time.Millisecond,
+		},
+		{
+			name:              "MoreThanDefaultBaseReachableTime",
+			baseReachableTime: 2 * defaultBaseReachableTime,
+			want:              2 * defaultBaseReachableTime,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			const nicID = 1
+
+			c := stack.DefaultNUDConfigurations()
+			c.BaseReachableTime = test.baseReachableTime
+
+			e := channel.New(0, 1280, linkAddr1)
+			e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+			s := stack.New(stack.Options{
+				// A neighbor cache is required to store NUDConfigurations. The
+				// networking stack will only allocate neighbor caches if a protocol
+				// providing link address resolution is specified (e.g. ARP or IPv6).
+				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				NUDConfigs:       c,
+			})
+			if err := s.CreateNIC(nicID, e); err != nil {
+				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+			}
+			sc, err := s.NUDConfigurations(nicID)
+			if err != nil {
+				t.Fatalf("got stack.NUDConfigurations(%d) = %s", nicID, err)
+			}
+			if got := sc.BaseReachableTime; got != test.want {
+				t.Errorf("got BaseReachableTime = %q, want = %q", got, test.want)
+			}
+		})
+	}
+}
+
+func TestNUDConfigurationsMinRandomFactor(t *testing.T) {
+	tests := []struct {
+		name            string
+		minRandomFactor float32
+		want            float32
+	}{
+		// Invalid cases
+		{
+			name:            "LessThanZero",
+			minRandomFactor: -1,
+			want:            defaultMinRandomFactor,
+		},
+		{
+			name:            "EqualToZero",
+			minRandomFactor: 0,
+			want:            defaultMinRandomFactor,
+		},
+		// Valid cases
+		{
+			name:            "MoreThanZero",
+			minRandomFactor: 1,
+			want:            1,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			const nicID = 1
+
+			c := stack.DefaultNUDConfigurations()
+			c.MinRandomFactor = test.minRandomFactor
+
+			e := channel.New(0, 1280, linkAddr1)
+			e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+			s := stack.New(stack.Options{
+				// A neighbor cache is required to store NUDConfigurations. The
+				// networking stack will only allocate neighbor caches if a protocol
+				// providing link address resolution is specified (e.g. ARP or IPv6).
+				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				NUDConfigs:       c,
+			})
+			if err := s.CreateNIC(nicID, e); err != nil {
+				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+			}
+			sc, err := s.NUDConfigurations(nicID)
+			if err != nil {
+				t.Fatalf("got stack.NUDConfigurations(%d) = %s", nicID, err)
+			}
+			if got := sc.MinRandomFactor; got != test.want {
+				t.Errorf("got MinRandomFactor = %f, want = %f", got, test.want)
+			}
+		})
+	}
+}
+
+func TestNUDConfigurationsMaxRandomFactor(t *testing.T) {
+	tests := []struct {
+		name            string
+		minRandomFactor float32
+		maxRandomFactor float32
+		want            float32
+	}{
+		// Invalid cases
+		{
+			name:            "LessThanZero",
+			minRandomFactor: defaultMinRandomFactor,
+			maxRandomFactor: -1,
+			want:            defaultMaxRandomFactor,
+		},
+		{
+			name:            "EqualToZero",
+			minRandomFactor: defaultMinRandomFactor,
+			maxRandomFactor: 0,
+			want:            defaultMaxRandomFactor,
+		},
+		{
+			name:            "LessThanMinRandomFactor",
+			minRandomFactor: defaultMinRandomFactor,
+			maxRandomFactor: defaultMinRandomFactor * 0.99,
+			want:            defaultMaxRandomFactor,
+		},
+		{
+			name:            "MoreThanMinRandomFactorWhenMinRandomFactorIsLargerThanMaxRandomFactorDefault",
+			minRandomFactor: defaultMaxRandomFactor * 2,
+			maxRandomFactor: defaultMaxRandomFactor,
+			want:            defaultMaxRandomFactor * 6,
+		},
+		// Valid cases
+		{
+			name:            "EqualToMinRandomFactor",
+			minRandomFactor: defaultMinRandomFactor,
+			maxRandomFactor: defaultMinRandomFactor,
+			want:            defaultMinRandomFactor,
+		},
+		{
+			name:            "MoreThanMinRandomFactor",
+			minRandomFactor: defaultMinRandomFactor,
+			maxRandomFactor: defaultMinRandomFactor * 1.1,
+			want:            defaultMinRandomFactor * 1.1,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			const nicID = 1
+
+			c := stack.DefaultNUDConfigurations()
+			c.MinRandomFactor = test.minRandomFactor
+			c.MaxRandomFactor = test.maxRandomFactor
+
+			e := channel.New(0, 1280, linkAddr1)
+			e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+			s := stack.New(stack.Options{
+				// A neighbor cache is required to store NUDConfigurations. The
+				// networking stack will only allocate neighbor caches if a protocol
+				// providing link address resolution is specified (e.g. ARP or IPv6).
+				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				NUDConfigs:       c,
+			})
+			if err := s.CreateNIC(nicID, e); err != nil {
+				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+			}
+			sc, err := s.NUDConfigurations(nicID)
+			if err != nil {
+				t.Fatalf("got stack.NUDConfigurations(%d) = %s", nicID, err)
+			}
+			if got := sc.MaxRandomFactor; got != test.want {
+				t.Errorf("got MaxRandomFactor = %f, want = %f", got, test.want)
+			}
+		})
+	}
+}
+
+func TestNUDConfigurationsRetransmitTimer(t *testing.T) {
+	tests := []struct {
+		name            string
+		retransmitTimer time.Duration
+		want            time.Duration
+	}{
+		// Invalid cases
+		{
+			name:            "EqualToZero",
+			retransmitTimer: 0,
+			want:            defaultRetransmitTimer,
+		},
+		{
+			name:            "LessThanMinimumRetransmitTimer",
+			retransmitTimer: minimumRetransmitTimer - time.Nanosecond,
+			want:            defaultRetransmitTimer,
+		},
+		// Valid cases
+		{
+			name:            "EqualToMinimumRetransmitTimer",
+			retransmitTimer: minimumRetransmitTimer,
+			want:            minimumBaseReachableTime,
+		},
+		{
+			name:            "LargetThanMinimumRetransmitTimer",
+			retransmitTimer: 2 * minimumBaseReachableTime,
+			want:            2 * minimumBaseReachableTime,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			const nicID = 1
+
+			c := stack.DefaultNUDConfigurations()
+			c.RetransmitTimer = test.retransmitTimer
+
+			e := channel.New(0, 1280, linkAddr1)
+			e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+			s := stack.New(stack.Options{
+				// A neighbor cache is required to store NUDConfigurations. The
+				// networking stack will only allocate neighbor caches if a protocol
+				// providing link address resolution is specified (e.g. ARP or IPv6).
+				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				NUDConfigs:       c,
+			})
+			if err := s.CreateNIC(nicID, e); err != nil {
+				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+			}
+			sc, err := s.NUDConfigurations(nicID)
+			if err != nil {
+				t.Fatalf("got stack.NUDConfigurations(%d) = %s", nicID, err)
+			}
+			if got := sc.RetransmitTimer; got != test.want {
+				t.Errorf("got RetransmitTimer = %q, want = %q", got, test.want)
+			}
+		})
+	}
+}
+
+func TestNUDConfigurationsDelayFirstProbeTime(t *testing.T) {
+	tests := []struct {
+		name                string
+		delayFirstProbeTime time.Duration
+		want                time.Duration
+	}{
+		// Invalid cases
+		{
+			name:                "EqualToZero",
+			delayFirstProbeTime: 0,
+			want:                defaultDelayFirstProbeTime,
+		},
+		// Valid cases
+		{
+			name:                "MoreThanZero",
+			delayFirstProbeTime: time.Millisecond,
+			want:                time.Millisecond,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			const nicID = 1
+
+			c := stack.DefaultNUDConfigurations()
+			c.DelayFirstProbeTime = test.delayFirstProbeTime
+
+			e := channel.New(0, 1280, linkAddr1)
+			e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+			s := stack.New(stack.Options{
+				// A neighbor cache is required to store NUDConfigurations. The
+				// networking stack will only allocate neighbor caches if a protocol
+				// providing link address resolution is specified (e.g. ARP or IPv6).
+				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				NUDConfigs:       c,
+			})
+			if err := s.CreateNIC(nicID, e); err != nil {
+				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+			}
+			sc, err := s.NUDConfigurations(nicID)
+			if err != nil {
+				t.Fatalf("got stack.NUDConfigurations(%d) = %s", nicID, err)
+			}
+			if got := sc.DelayFirstProbeTime; got != test.want {
+				t.Errorf("got DelayFirstProbeTime = %q, want = %q", got, test.want)
+			}
+		})
+	}
+}
+
+func TestNUDConfigurationsMaxMulticastProbes(t *testing.T) {
+	tests := []struct {
+		name               string
+		maxMulticastProbes uint32
+		want               uint32
+	}{
+		// Invalid cases
+		{
+			name:               "EqualToZero",
+			maxMulticastProbes: 0,
+			want:               defaultMaxMulticastProbes,
+		},
+		// Valid cases
+		{
+			name:               "MoreThanZero",
+			maxMulticastProbes: 1,
+			want:               1,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			const nicID = 1
+
+			c := stack.DefaultNUDConfigurations()
+			c.MaxMulticastProbes = test.maxMulticastProbes
+
+			e := channel.New(0, 1280, linkAddr1)
+			e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+			s := stack.New(stack.Options{
+				// A neighbor cache is required to store NUDConfigurations. The
+				// networking stack will only allocate neighbor caches if a protocol
+				// providing link address resolution is specified (e.g. ARP or IPv6).
+				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				NUDConfigs:       c,
+			})
+			if err := s.CreateNIC(nicID, e); err != nil {
+				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+			}
+			sc, err := s.NUDConfigurations(nicID)
+			if err != nil {
+				t.Fatalf("got stack.NUDConfigurations(%d) = %s", nicID, err)
+			}
+			if got := sc.MaxMulticastProbes; got != test.want {
+				t.Errorf("got MaxMulticastProbes = %q, want = %q", got, test.want)
+			}
+		})
+	}
+}
+
+func TestNUDConfigurationsMaxUnicastProbes(t *testing.T) {
+	tests := []struct {
+		name             string
+		maxUnicastProbes uint32
+		want             uint32
+	}{
+		// Invalid cases
+		{
+			name:             "EqualToZero",
+			maxUnicastProbes: 0,
+			want:             defaultMaxUnicastProbes,
+		},
+		// Valid cases
+		{
+			name:             "MoreThanZero",
+			maxUnicastProbes: 1,
+			want:             1,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			const nicID = 1
+
+			c := stack.DefaultNUDConfigurations()
+			c.MaxUnicastProbes = test.maxUnicastProbes
+
+			e := channel.New(0, 1280, linkAddr1)
+			e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+			s := stack.New(stack.Options{
+				// A neighbor cache is required to store NUDConfigurations. The
+				// networking stack will only allocate neighbor caches if a protocol
+				// providing link address resolution is specified (e.g. ARP or IPv6).
+				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				NUDConfigs:       c,
+			})
+			if err := s.CreateNIC(nicID, e); err != nil {
+				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+			}
+			sc, err := s.NUDConfigurations(nicID)
+			if err != nil {
+				t.Fatalf("got stack.NUDConfigurations(%d) = %s", nicID, err)
+			}
+			if got := sc.MaxUnicastProbes; got != test.want {
+				t.Errorf("got MaxUnicastProbes = %q, want = %q", got, test.want)
+			}
+		})
+	}
+}
+
+func TestNUDConfigurationsUnreachableTime(t *testing.T) {
+	tests := []struct {
+		name            string
+		unreachableTime time.Duration
+		want            time.Duration
+	}{
+		// Invalid cases
+		{
+			name:            "EqualToZero",
+			unreachableTime: 0,
+			want:            defaultUnreachableTime,
+		},
+		// Valid cases
+		{
+			name:            "MoreThanZero",
+			unreachableTime: time.Millisecond,
+			want:            time.Millisecond,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			const nicID = 1
+
+			c := stack.DefaultNUDConfigurations()
+			c.UnreachableTime = test.unreachableTime
+
+			e := channel.New(0, 1280, linkAddr1)
+			e.LinkEPCapabilities |= stack.CapabilityResolutionRequired
+
+			s := stack.New(stack.Options{
+				// A neighbor cache is required to store NUDConfigurations. The
+				// networking stack will only allocate neighbor caches if a protocol
+				// providing link address resolution is specified (e.g. ARP or IPv6).
+				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				NUDConfigs:       c,
+			})
+			if err := s.CreateNIC(nicID, e); err != nil {
+				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
+			}
+			sc, err := s.NUDConfigurations(nicID)
+			if err != nil {
+				t.Fatalf("got stack.NUDConfigurations(%d) = %s", nicID, err)
+			}
+			if got := sc.UnreachableTime; got != test.want {
+				t.Errorf("got UnreachableTime = %q, want = %q", got, test.want)
+			}
+		})
+	}
+}
+
+// TestNUDStateReachableTime verifies the correctness of the ReachableTime
+// computation.
+func TestNUDStateReachableTime(t *testing.T) {
+	tests := []struct {
+		name              string
+		baseReachableTime time.Duration
+		minRandomFactor   float32
+		maxRandomFactor   float32
+		want              time.Duration
+	}{
+		{
+			name:              "AllZeros",
+			baseReachableTime: 0,
+			minRandomFactor:   0,
+			maxRandomFactor:   0,
+			want:              0,
+		},
+		{
+			name:              "ZeroMaxRandomFactor",
+			baseReachableTime: time.Second,
+			minRandomFactor:   0,
+			maxRandomFactor:   0,
+			want:              0,
+		},
+		{
+			name:              "ZeroMinRandomFactor",
+			baseReachableTime: time.Second,
+			minRandomFactor:   0,
+			maxRandomFactor:   1,
+			want:              time.Duration(defaultFakeRandomNum * float32(time.Second)),
+		},
+		{
+			name:              "FractionalRandomFactor",
+			baseReachableTime: time.Duration(math.MaxInt64),
+			minRandomFactor:   0.001,
+			maxRandomFactor:   0.002,
+			want:              time.Duration((0.001 + (0.001 * defaultFakeRandomNum)) * float32(math.MaxInt64)),
+		},
+		{
+			name:              "MinAndMaxRandomFactorsEqual",
+			baseReachableTime: time.Second,
+			minRandomFactor:   1,
+			maxRandomFactor:   1,
+			want:              time.Second,
+		},
+		{
+			name:              "MinAndMaxRandomFactorsDifferent",
+			baseReachableTime: time.Second,
+			minRandomFactor:   1,
+			maxRandomFactor:   2,
+			want:              time.Duration((1.0 + defaultFakeRandomNum) * float32(time.Second)),
+		},
+		{
+			name:              "MaxInt64",
+			baseReachableTime: time.Duration(math.MaxInt64),
+			minRandomFactor:   1,
+			maxRandomFactor:   1,
+			want:              time.Duration(math.MaxInt64),
+		},
+		{
+			name:              "Overflow",
+			baseReachableTime: time.Duration(math.MaxInt64),
+			minRandomFactor:   1.5,
+			maxRandomFactor:   1.5,
+			want:              time.Duration(math.MaxInt64),
+		},
+		{
+			name:              "DoubleOverflow",
+			baseReachableTime: time.Duration(math.MaxInt64),
+			minRandomFactor:   2.5,
+			maxRandomFactor:   2.5,
+			want:              time.Duration(math.MaxInt64),
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			c := stack.NUDConfigurations{
+				BaseReachableTime: test.baseReachableTime,
+				MinRandomFactor:   test.minRandomFactor,
+				MaxRandomFactor:   test.maxRandomFactor,
+			}
+			// A fake random number generator is used to ensure deterministic
+			// results.
+			rng := fakeRand{
+				num: defaultFakeRandomNum,
+			}
+			s := stack.NewNUDState(c, &rng)
+			if got, want := s.ReachableTime(), test.want; got != want {
+				t.Errorf("got ReachableTime = %q, want = %q", got, want)
+			}
+		})
+	}
+}
+
+// TestNUDStateRecomputeReachableTime exercises the ReachableTime function
+// twice to verify recomputation of reachable time when the min random factor,
+// max random factor, or base reachable time changes.
+func TestNUDStateRecomputeReachableTime(t *testing.T) {
+	const defaultBase = time.Second
+	const defaultMin = 2.0 * defaultMaxRandomFactor
+	const defaultMax = 3.0 * defaultMaxRandomFactor
+
+	tests := []struct {
+		name              string
+		baseReachableTime time.Duration
+		minRandomFactor   float32
+		maxRandomFactor   float32
+		want              time.Duration
+	}{
+		{
+			name:              "BaseReachableTime",
+			baseReachableTime: 2 * defaultBase,
+			minRandomFactor:   defaultMin,
+			maxRandomFactor:   defaultMax,
+			want:              time.Duration((defaultMin + (defaultMax-defaultMin)*defaultFakeRandomNum) * float32(2*defaultBase)),
+		},
+		{
+			name:              "MinRandomFactor",
+			baseReachableTime: defaultBase,
+			minRandomFactor:   defaultMax,
+			maxRandomFactor:   defaultMax,
+			want:              time.Duration(defaultMax * float32(defaultBase)),
+		},
+		{
+			name:              "MaxRandomFactor",
+			baseReachableTime: defaultBase,
+			minRandomFactor:   defaultMin,
+			maxRandomFactor:   defaultMin,
+			want:              time.Duration(defaultMin * float32(defaultBase)),
+		},
+		{
+			name:              "BothRandomFactor",
+			baseReachableTime: defaultBase,
+			minRandomFactor:   2 * defaultMin,
+			maxRandomFactor:   2 * defaultMax,
+			want:              time.Duration((2*defaultMin + (2*defaultMax-2*defaultMin)*defaultFakeRandomNum) * float32(defaultBase)),
+		},
+		{
+			name:              "BaseReachableTimeAndBothRandomFactors",
+			baseReachableTime: 2 * defaultBase,
+			minRandomFactor:   2 * defaultMin,
+			maxRandomFactor:   2 * defaultMax,
+			want:              time.Duration((2*defaultMin + (2*defaultMax-2*defaultMin)*defaultFakeRandomNum) * float32(2*defaultBase)),
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			c := stack.DefaultNUDConfigurations()
+			c.BaseReachableTime = defaultBase
+			c.MinRandomFactor = defaultMin
+			c.MaxRandomFactor = defaultMax
+
+			// A fake random number generator is used to ensure deterministic
+			// results.
+			rng := fakeRand{
+				num: defaultFakeRandomNum,
+			}
+			s := stack.NewNUDState(c, &rng)
+			old := s.ReachableTime()
+
+			if got, want := s.ReachableTime(), old; got != want {
+				t.Errorf("got ReachableTime = %q, want = %q", got, want)
+			}
+
+			// Check for recomputation when changing the min random factor, the max
+			// random factor, the base reachability time, or any permutation of those
+			// three options.
+			c.BaseReachableTime = test.baseReachableTime
+			c.MinRandomFactor = test.minRandomFactor
+			c.MaxRandomFactor = test.maxRandomFactor
+			s.SetConfig(c)
+
+			if got, want := s.ReachableTime(), test.want; got != want {
+				t.Errorf("got ReachableTime = %q, want = %q", got, want)
+			}
+
+			// Verify that ReachableTime isn't recomputed when none of the
+			// configuration options change. The random factor is changed so that if
+			// a recompution were to occur, ReachableTime would change.
+			rng.num = defaultFakeRandomNum / 2.0
+			if got, want := s.ReachableTime(), test.want; got != want {
+				t.Errorf("got ReachableTime = %q, want = %q", got, want)
+			}
+		})
+	}
+}
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 36626e051..3f07e4159 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -445,6 +445,9 @@ type Stack struct {
 	// ndpConfigs is the default NDP configurations used by interfaces.
 	ndpConfigs NDPConfigurations
 
+	// nudConfigs is the default NUD configurations used by interfaces.
+	nudConfigs NUDConfigurations
+
 	// autoGenIPv6LinkLocal determines whether or not the stack will attempt
 	// to auto-generate an IPv6 link-local address for newly enabled non-loopback
 	// NICs. See the AutoGenIPv6LinkLocal field of Options for more details.
@@ -454,6 +457,10 @@ type Stack struct {
 	// integrator NDP related events.
 	ndpDisp NDPDispatcher
 
+	// nudDisp is the NUD event dispatcher that is used to send the netstack
+	// integrator NUD related events.
+	nudDisp NUDDispatcher
+
 	// uniqueIDGenerator is a generator of unique identifiers.
 	uniqueIDGenerator UniqueID
 
@@ -518,6 +525,9 @@ type Options struct {
 	// before assigning an address to a NIC.
 	NDPConfigs NDPConfigurations
 
+	// NUDConfigs is the default NUD configurations used by interfaces.
+	NUDConfigs NUDConfigurations
+
 	// AutoGenIPv6LinkLocal determines whether or not the stack will attempt to
 	// auto-generate an IPv6 link-local address for newly enabled non-loopback
 	// NICs.
@@ -536,6 +546,10 @@ type Options struct {
 	// receive NDP related events.
 	NDPDisp NDPDispatcher
 
+	// NUDDisp is the NUD event dispatcher that an integrator can provide to
+	// receive NUD related events.
+	NUDDisp NUDDispatcher
+
 	// RawFactory produces raw endpoints. Raw endpoints are enabled only if
 	// this is non-nil.
 	RawFactory RawFactory
@@ -670,6 +684,8 @@ func New(opts Options) *Stack {
 	// Make sure opts.NDPConfigs contains valid values only.
 	opts.NDPConfigs.validate()
 
+	opts.NUDConfigs.resetInvalidFields()
+
 	s := &Stack{
 		transportProtocols:   make(map[tcpip.TransportProtocolNumber]*transportProtocolState),
 		networkProtocols:     make(map[tcpip.NetworkProtocolNumber]NetworkProtocol),
@@ -685,9 +701,11 @@ func New(opts Options) *Stack {
 		icmpRateLimiter:      NewICMPRateLimiter(),
 		seed:                 generateRandUint32(),
 		ndpConfigs:           opts.NDPConfigs,
+		nudConfigs:           opts.NUDConfigs,
 		autoGenIPv6LinkLocal: opts.AutoGenIPv6LinkLocal,
 		uniqueIDGenerator:    opts.UniqueID,
 		ndpDisp:              opts.NDPDisp,
+		nudDisp:              opts.NUDDisp,
 		opaqueIIDOpts:        opts.OpaqueIIDOpts,
 		tempIIDSeed:          opts.TempIIDSeed,
 		forwarder:            newForwardQueue(),
@@ -1869,10 +1887,38 @@ func (s *Stack) SetNDPConfigurations(id tcpip.NICID, c NDPConfigurations) *tcpip
 	}
 
 	nic.setNDPConfigs(c)
-
 	return nil
 }
 
+// NUDConfigurations gets the per-interface NUD configurations.
+func (s *Stack) NUDConfigurations(id tcpip.NICID) (NUDConfigurations, *tcpip.Error) {
+	s.mu.RLock()
+	nic, ok := s.nics[id]
+	s.mu.RUnlock()
+
+	if !ok {
+		return NUDConfigurations{}, tcpip.ErrUnknownNICID
+	}
+
+	return nic.NUDConfigs()
+}
+
+// SetNUDConfigurations sets the per-interface NUD configurations.
+//
+// Note, if c contains invalid NUD configuration values, it will be fixed to
+// use default values for the erroneous values.
+func (s *Stack) SetNUDConfigurations(id tcpip.NICID, c NUDConfigurations) *tcpip.Error {
+	s.mu.RLock()
+	nic, ok := s.nics[id]
+	s.mu.RUnlock()
+
+	if !ok {
+		return tcpip.ErrUnknownNICID
+	}
+
+	return nic.setNUDConfigs(c)
+}
+
 // HandleNDPRA provides a NIC with ID id a validated NDP Router Advertisement
 // message that it needs to handle.
 func (s *Stack) HandleNDPRA(id tcpip.NICID, ip tcpip.Address, ra header.NDPRouterAdvert) *tcpip.Error {