[management] Skip full network map on Sync when peer state is unchanged

Introduce a peer-sync cache keyed by WireGuard pubkey that records the
NetworkMap.Serial and meta hash the server last delivered to each peer.
When a Sync request arrives from a non-Android peer whose cached serial
matches the current account serial and whose meta hash matches the last
delivery, short-circuit SyncAndMarkPeer and reply with a NetbirdConfig-only
SyncResponse mirroring the shape TimeBasedAuthSecretsManager already pushes
for TURN/Relay token rotation. The client keeps its existing network map
state and refreshes only control-plane credentials.

The fast path avoids GetAccountWithBackpressure, the full per-peer map
assembly, posture-check recomputation and the large encrypted payload on
every reconnect of a peer whose account is quiescent. Slow path remains
the source of truth for any real state change; every full-map send (initial
sync or streamed NetworkMap update) rewrites the cache, and every Login
deletes it so a fresh map is guaranteed after SSH key rotation, approval
changes or re-registration.

Backend-only: no proto changes and no client changes. Compatibility is
provided by the existing client handling of nil NetworkMap in handleSync
(every version from v0.20.0 on). Android is gated out at the server because
its readInitialSettings path calls GrpcClient.GetNetworkMap which errors on
nil map. The cache is wired through BaseServer.CacheStore() so it shares
the same Redis/in-memory backend as OneTimeTokenStore and PKCEVerifierStore.

Test coverage lands in four layers:
- Pure decision function (peer_serial_cache_decision_test.go)
- Cache wrapper with TTL + concurrency (peer_serial_cache_test.go)
- Response shape unit tests (sync_fast_path_response_test.go)
- In-process gRPC behavioural tests covering first sync, reconnect skip,
  android never-skip, meta change, login invalidation, and serial advance
  (management/server/sync_fast_path_test.go)
- Frozen SyncRequest wire-format fixtures for v0.20.0 / v0.40.0 / v0.60.0
  / current / android replayed against the in-process server
  (management/server/sync_legacy_wire_test.go + testdata fixtures)

management/internals/shared/grpc/peer_serial_cache.go

@@ -0,0 +1,89 @@
+package grpc
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"time"
+
+	"github.com/eko/gocache/lib/v4/cache"
+	"github.com/eko/gocache/lib/v4/store"
+	log "github.com/sirupsen/logrus"
+)
+
+const (
+	peerSerialCacheKeyPrefix = "peer-sync:"
+
+	// DefaultPeerSerialCacheTTL bounds how long a cached serial survives. If the
+	// cache write on a full-map send ever drops, entries naturally expire and
+	// the next Sync falls back to the full path, re-priming the cache.
+	DefaultPeerSerialCacheTTL = 24 * time.Hour
+)
+
+// PeerSerialCache records the NetworkMap serial and meta hash last delivered to
+// each peer on Sync. Lookups are used to skip full network map computation when
+// the peer already has the latest state. Backed by the shared cache store so
+// entries survive management replicas sharing a Redis instance.
+type PeerSerialCache struct {
+	cache *cache.Cache[string]
+	ctx   context.Context
+	ttl   time.Duration
+}
+
+// NewPeerSerialCache creates a cache wrapper bound to the shared cache store.
+// The ttl is applied to every Set call; entries older than ttl are treated as
+// misses so the server eventually converges to delivering a full map even if
+// an earlier Set was lost.
+func NewPeerSerialCache(ctx context.Context, cacheStore store.StoreInterface, ttl time.Duration) *PeerSerialCache {
+	return &PeerSerialCache{
+		cache: cache.New[string](cacheStore),
+		ctx:   ctx,
+		ttl:   ttl,
+	}
+}
+
+// Get returns the entry previously recorded for this peer and whether a valid
+// entry was found. A cache miss or any read error is reported as a miss so
+// callers fall back to the full map path.
+func (c *PeerSerialCache) Get(pubKey string) (peerSyncEntry, bool) {
+	raw, err := c.cache.Get(c.ctx, peerSerialCacheKeyPrefix+pubKey)
+	if err != nil {
+		return peerSyncEntry{}, false
+	}
+
+	entry := peerSyncEntry{}
+	if err := json.Unmarshal([]byte(raw), &entry); err != nil {
+		log.Debugf("peer serial cache: unmarshal entry for %s: %v", pubKey, err)
+		return peerSyncEntry{}, false
+	}
+	return entry, true
+}
+
+// Set records what the server most recently delivered to this peer. Errors are
+// logged at debug level so cache outages degrade gracefully into the full map
+// path on the next Sync rather than failing the current Sync.
+func (c *PeerSerialCache) Set(pubKey string, entry peerSyncEntry) {
+	payload, err := json.Marshal(entry)
+	if err != nil {
+		log.Debugf("peer serial cache: marshal entry for %s: %v", pubKey, err)
+		return
+	}
+
+	if err := c.cache.Set(c.ctx, peerSerialCacheKeyPrefix+pubKey, string(payload), store.WithExpiration(c.ttl)); err != nil {
+		log.Debugf("peer serial cache: set entry for %s: %v", pubKey, err)
+	}
+}
+
+// Delete removes any cached entry for this peer. Used on Login so the next
+// Sync always sees a miss and delivers a full map.
+func (c *PeerSerialCache) Delete(pubKey string) {
+	if err := c.cache.Delete(c.ctx, peerSerialCacheKeyPrefix+pubKey); err != nil {
+		log.Debugf("peer serial cache: delete entry for %s: %v", pubKey, err)
+	}
+}
+
+// cacheKey exposes the namespaced key for tests that need to peek at the raw
+// storage, e.g. when asserting TTL behaviour against Redis.
+func (c *PeerSerialCache) cacheKey(pubKey string) string {
+	return fmt.Sprintf("%s%s", peerSerialCacheKeyPrefix, pubKey)
+}

management/internals/shared/grpc/peer_serial_cache_decision_test.go

@@ -0,0 +1,116 @@
+package grpc
+
+import "testing"
+
+func TestShouldSkipNetworkMap(t *testing.T) {
+	tests := []struct {
+		name          string
+		goOS          string
+		hit           bool
+		cached        peerSyncEntry
+		currentSerial uint64
+		incomingMeta  uint64
+		want          bool
+	}{
+		{
+			name:          "android never skips even on clean cache hit",
+			goOS:          "android",
+			hit:           true,
+			cached:        peerSyncEntry{Serial: 42, MetaHash: 7},
+			currentSerial: 42,
+			incomingMeta:  7,
+			want:          false,
+		},
+		{
+			name:          "android uppercase never skips",
+			goOS:          "Android",
+			hit:           true,
+			cached:        peerSyncEntry{Serial: 42, MetaHash: 7},
+			currentSerial: 42,
+			incomingMeta:  7,
+			want:          false,
+		},
+		{
+			name:          "cache miss forces full path",
+			goOS:          "linux",
+			hit:           false,
+			cached:        peerSyncEntry{},
+			currentSerial: 42,
+			incomingMeta:  7,
+			want:          false,
+		},
+		{
+			name:          "serial mismatch forces full path",
+			goOS:          "linux",
+			hit:           true,
+			cached:        peerSyncEntry{Serial: 41, MetaHash: 7},
+			currentSerial: 42,
+			incomingMeta:  7,
+			want:          false,
+		},
+		{
+			name:          "meta mismatch forces full path",
+			goOS:          "linux",
+			hit:           true,
+			cached:        peerSyncEntry{Serial: 42, MetaHash: 7},
+			currentSerial: 42,
+			incomingMeta:  9,
+			want:          false,
+		},
+		{
+			name:          "clean hit on linux skips",
+			goOS:          "linux",
+			hit:           true,
+			cached:        peerSyncEntry{Serial: 42, MetaHash: 7},
+			currentSerial: 42,
+			incomingMeta:  7,
+			want:          true,
+		},
+		{
+			name:          "clean hit on darwin skips",
+			goOS:          "darwin",
+			hit:           true,
+			cached:        peerSyncEntry{Serial: 42, MetaHash: 7},
+			currentSerial: 42,
+			incomingMeta:  7,
+			want:          true,
+		},
+		{
+			name:          "clean hit on windows skips",
+			goOS:          "windows",
+			hit:           true,
+			cached:        peerSyncEntry{Serial: 42, MetaHash: 7},
+			currentSerial: 42,
+			incomingMeta:  7,
+			want:          true,
+		},
+		{
+			name:          "zero current serial never skips",
+			goOS:          "linux",
+			hit:           true,
+			cached:        peerSyncEntry{Serial: 0, MetaHash: 7},
+			currentSerial: 0,
+			incomingMeta:  7,
+			want:          false,
+		},
+		{
+			name:          "empty goos treated as non-android and skips",
+			goOS:          "",
+			hit:           true,
+			cached:        peerSyncEntry{Serial: 42, MetaHash: 7},
+			currentSerial: 42,
+			incomingMeta:  7,
+			want:          true,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := shouldSkipNetworkMap(tc.goOS, tc.hit, tc.cached, tc.currentSerial, tc.incomingMeta)
+			if got != tc.want {
+				t.Fatalf("shouldSkipNetworkMap(%q, hit=%v, cached=%+v, current=%d, meta=%d) = %v, want %v",
+					tc.goOS, tc.hit, tc.cached, tc.currentSerial, tc.incomingMeta, got, tc.want)
+			}
+		})
+	}
+}

management/internals/shared/grpc/peer_serial_cache_test.go

@@ -0,0 +1,134 @@
+package grpc
+
+import (
+	"context"
+	"os"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	nbcache "github.com/netbirdio/netbird/management/server/cache"
+)
+
+func newTestPeerSerialCache(t *testing.T, ttl, cleanup time.Duration) *PeerSerialCache {
+	t.Helper()
+	s, err := nbcache.NewStore(context.Background(), ttl, cleanup, 100)
+	require.NoError(t, err, "cache store must initialise")
+	return NewPeerSerialCache(context.Background(), s, ttl)
+}
+
+func TestPeerSerialCache_GetSetDelete(t *testing.T) {
+	c := newTestPeerSerialCache(t, time.Minute, time.Minute)
+	key := "pubkey-aaa"
+
+	_, hit := c.Get(key)
+	assert.False(t, hit, "empty cache must miss")
+
+	c.Set(key, peerSyncEntry{Serial: 42, MetaHash: 7})
+
+	entry, hit := c.Get(key)
+	require.True(t, hit, "after Set, Get must hit")
+	assert.Equal(t, uint64(42), entry.Serial, "serial roundtrip")
+	assert.Equal(t, uint64(7), entry.MetaHash, "meta hash roundtrip")
+
+	c.Delete(key)
+	_, hit = c.Get(key)
+	assert.False(t, hit, "after Delete, Get must miss")
+}
+
+func TestPeerSerialCache_GetMissReturnsZero(t *testing.T) {
+	c := newTestPeerSerialCache(t, time.Minute, time.Minute)
+
+	entry, hit := c.Get("missing")
+	assert.False(t, hit, "miss must report false")
+	assert.Equal(t, peerSyncEntry{}, entry, "miss must return zero value")
+}
+
+func TestPeerSerialCache_TTLExpiry(t *testing.T) {
+	c := newTestPeerSerialCache(t, 100*time.Millisecond, 10*time.Millisecond)
+	key := "pubkey-ttl"
+
+	c.Set(key, peerSyncEntry{Serial: 1, MetaHash: 1})
+	time.Sleep(250 * time.Millisecond)
+
+	_, hit := c.Get(key)
+	assert.False(t, hit, "entry must expire after TTL")
+}
+
+func TestPeerSerialCache_OverwriteUpdatesValue(t *testing.T) {
+	c := newTestPeerSerialCache(t, time.Minute, time.Minute)
+	key := "pubkey-overwrite"
+
+	c.Set(key, peerSyncEntry{Serial: 1, MetaHash: 1})
+	c.Set(key, peerSyncEntry{Serial: 99, MetaHash: 123})
+
+	entry, hit := c.Get(key)
+	require.True(t, hit, "overwritten key must still be present")
+	assert.Equal(t, uint64(99), entry.Serial, "overwrite updates serial")
+	assert.Equal(t, uint64(123), entry.MetaHash, "overwrite updates meta hash")
+}
+
+func TestPeerSerialCache_IsolatedPerKey(t *testing.T) {
+	c := newTestPeerSerialCache(t, time.Minute, time.Minute)
+
+	c.Set("a", peerSyncEntry{Serial: 1, MetaHash: 1})
+	c.Set("b", peerSyncEntry{Serial: 2, MetaHash: 2})
+
+	a, hitA := c.Get("a")
+	b, hitB := c.Get("b")
+	require.True(t, hitA, "key a must hit")
+	require.True(t, hitB, "key b must hit")
+	assert.Equal(t, uint64(1), a.Serial, "key a serial")
+	assert.Equal(t, uint64(2), b.Serial, "key b serial")
+
+	c.Delete("a")
+	_, hitA = c.Get("a")
+	_, hitB = c.Get("b")
+	assert.False(t, hitA, "deleting a must not affect b")
+	assert.True(t, hitB, "b must remain after a delete")
+}
+
+func TestPeerSerialCache_Concurrent(t *testing.T) {
+	c := newTestPeerSerialCache(t, time.Minute, time.Minute)
+
+	var wg sync.WaitGroup
+	const workers = 50
+	const iterations = 20
+
+	for w := 0; w < workers; w++ {
+		wg.Add(1)
+		go func(id int) {
+			defer wg.Done()
+			key := "pubkey"
+			for i := 0; i < iterations; i++ {
+				c.Set(key, peerSyncEntry{Serial: uint64(id*iterations + i), MetaHash: uint64(id)})
+				_, _ = c.Get(key)
+			}
+		}(w)
+	}
+
+	wg.Wait()
+
+	_, hit := c.Get("pubkey")
+	assert.True(t, hit, "cache must survive concurrent Set/Get without deadlock")
+}
+
+func TestPeerSerialCache_Redis(t *testing.T) {
+	if os.Getenv(nbcache.RedisStoreEnvVar) == "" {
+		t.Skipf("set %s to run this test against a real Redis", nbcache.RedisStoreEnvVar)
+	}
+
+	s, err := nbcache.NewStore(context.Background(), time.Minute, 10*time.Second, 10)
+	require.NoError(t, err, "redis store must initialise")
+	c := NewPeerSerialCache(context.Background(), s, time.Minute)
+
+	key := "redis-pubkey"
+	c.Set(key, peerSyncEntry{Serial: 42, MetaHash: 7})
+	entry, hit := c.Get(key)
+	require.True(t, hit, "redis hit expected")
+	assert.Equal(t, uint64(42), entry.Serial)
+	c.Delete(key)
+}

management/internals/shared/grpc/server.go

@@ -84,9 +84,15 @@ type Server struct {
 
 	reverseProxyManager rpservice.Manager
 	reverseProxyMu      sync.RWMutex
+
+	// peerSerialCache lets Sync skip full network map computation when the peer
+	// already has the latest account serial. A nil cache disables the fast path.
+	peerSerialCache *PeerSerialCache
 }
 
-// NewServer creates a new Management server
+// NewServer creates a new Management server. peerSerialCache is optional; when
+// nil the Sync fast path is disabled and every request runs the full map
+// computation, matching the pre-cache behaviour.
 func NewServer(
 	config *nbconfig.Config,
 	accountManager account.Manager,
@@ -98,6 +104,7 @@ func NewServer(
 	integratedPeerValidator integrated_validator.IntegratedValidator,
 	networkMapController network_map.Controller,
 	oAuthConfigProvider idp.OAuthConfigProvider,
+	peerSerialCache *PeerSerialCache,
 ) (*Server, error) {
 	if appMetrics != nil {
 		// update gauge based on number of connected peers which is equal to open gRPC streams
@@ -145,6 +152,8 @@ func NewServer(
 
 		syncLim:        syncLim,
 		syncLimEnabled: syncLimEnabled,
+
+		peerSerialCache: peerSerialCache,
 	}, nil
 }
 
@@ -305,6 +314,10 @@ func (s *Server) Sync(req *proto.EncryptedMessage, srv proto.ManagementService_S
 	metahash := metaHash(peerMeta, realIP.String())
 	s.loginFilter.addLogin(peerKey.String(), metahash)
 
+	if took, err := s.tryFastPathSync(ctx, reqStart, syncStart, accountID, peerKey, peerMeta, realIP, metahash, srv, &unlock); took {
+		return err
+	}
+
 	peer, netMap, postureChecks, dnsFwdPort, err := s.accountManager.SyncAndMarkPeer(ctx, accountID, peerKey.String(), peerMeta, realIP, syncStart)
 	if err != nil {
 		log.WithContext(ctx).Debugf("error while syncing peer %s: %v", peerKey.String(), err)
@@ -319,6 +332,7 @@ func (s *Server) Sync(req *proto.EncryptedMessage, srv proto.ManagementService_S
 		s.cancelPeerRoutinesWithoutLock(ctx, accountID, peer, syncStart)
 		return err
 	}
+	s.recordPeerSyncEntry(peerKey.String(), netMap, metahash)
 
 	updates, err := s.networkMapController.OnPeerConnected(ctx, accountID, peer.ID)
 	if err != nil {
@@ -340,7 +354,7 @@ func (s *Server) Sync(req *proto.EncryptedMessage, srv proto.ManagementService_S
 
 	s.syncSem.Add(-1)
 
-	return s.handleUpdates(ctx, accountID, peerKey, peer, updates, srv, syncStart)
+	return s.handleUpdates(ctx, accountID, peerKey, peer, metahash, updates, srv, syncStart)
 }
 
 func (s *Server) handleHandshake(ctx context.Context, srv proto.ManagementService_JobServer) (wgtypes.Key, error) {
@@ -410,8 +424,9 @@ func (s *Server) sendJobsLoop(ctx context.Context, accountID string, peerKey wgt
 // handleUpdates sends updates to the connected peer until the updates channel is closed.
 // It implements a backpressure mechanism that sends the first update immediately,
 // then debounces subsequent rapid updates, ensuring only the latest update is sent
-// after a quiet period.
-func (s *Server) handleUpdates(ctx context.Context, accountID string, peerKey wgtypes.Key, peer *nbpeer.Peer, updates chan *network_map.UpdateMessage, srv proto.ManagementService_SyncServer, streamStartTime time.Time) error {
+// after a quiet period. peerMetaHash is forwarded to sendUpdate so the peer-sync
+// cache can record the serial this peer just received.
+func (s *Server) handleUpdates(ctx context.Context, accountID string, peerKey wgtypes.Key, peer *nbpeer.Peer, peerMetaHash uint64, updates chan *network_map.UpdateMessage, srv proto.ManagementService_SyncServer, streamStartTime time.Time) error {
 	log.WithContext(ctx).Tracef("starting to handle updates for peer %s", peerKey.String())
 
 	// Create a debouncer for this peer connection
@@ -436,7 +451,7 @@ func (s *Server) handleUpdates(ctx context.Context, accountID string, peerKey wg
 			log.WithContext(ctx).Debugf("received an update for peer %s", peerKey.String())
 			if debouncer.ProcessUpdate(update) {
 				// Send immediately (first update or after quiet period)
-				if err := s.sendUpdate(ctx, accountID, peerKey, peer, update, srv, streamStartTime); err != nil {
+				if err := s.sendUpdate(ctx, accountID, peerKey, peer, peerMetaHash, update, srv, streamStartTime); err != nil {
 					log.WithContext(ctx).Debugf("error while sending an update to peer %s: %v", peerKey.String(), err)
 					return err
 				}
@@ -450,7 +465,7 @@ func (s *Server) handleUpdates(ctx context.Context, accountID string, peerKey wg
 			}
 			log.WithContext(ctx).Debugf("sending %d debounced update(s) for peer %s", len(pendingUpdates), peerKey.String())
 			for _, pendingUpdate := range pendingUpdates {
-				if err := s.sendUpdate(ctx, accountID, peerKey, peer, pendingUpdate, srv, streamStartTime); err != nil {
+				if err := s.sendUpdate(ctx, accountID, peerKey, peer, peerMetaHash, pendingUpdate, srv, streamStartTime); err != nil {
 					log.WithContext(ctx).Debugf("error while sending an update to peer %s: %v", peerKey.String(), err)
 					return err
 				}
@@ -468,7 +483,9 @@ func (s *Server) handleUpdates(ctx context.Context, accountID string, peerKey wg
 
 // sendUpdate encrypts the update message using the peer key and the server's wireguard key,
 // then sends the encrypted message to the connected peer via the sync server.
-func (s *Server) sendUpdate(ctx context.Context, accountID string, peerKey wgtypes.Key, peer *nbpeer.Peer, update *network_map.UpdateMessage, srv proto.ManagementService_SyncServer, streamStartTime time.Time) error {
+// For MessageTypeNetworkMap updates it records the delivered serial in the
+// peer-sync cache so a subsequent Sync with the same serial can take the fast path.
+func (s *Server) sendUpdate(ctx context.Context, accountID string, peerKey wgtypes.Key, peer *nbpeer.Peer, peerMetaHash uint64, update *network_map.UpdateMessage, srv proto.ManagementService_SyncServer, streamStartTime time.Time) error {
 	key, err := s.secretsManager.GetWGKey()
 	if err != nil {
 		s.cancelPeerRoutines(ctx, accountID, peer, streamStartTime)
@@ -488,6 +505,9 @@ func (s *Server) sendUpdate(ctx context.Context, accountID string, peerKey wgtyp
 		s.cancelPeerRoutines(ctx, accountID, peer, streamStartTime)
 		return status.Errorf(codes.Internal, "failed sending update message")
 	}
+	if update.MessageType == network_map.MessageTypeNetworkMap {
+		s.recordPeerSyncEntryFromUpdate(peerKey.String(), update, peerMetaHash)
+	}
 	log.WithContext(ctx).Debugf("sent an update to peer %s", peerKey.String())
 	return nil
 }
@@ -772,6 +792,7 @@ func (s *Server) Login(ctx context.Context, req *proto.EncryptedMessage) (*proto
 		log.WithContext(ctx).Warnf("failed logging in peer %s: %s", peerKey, err)
 		return nil, mapError(ctx, err)
 	}
+	s.invalidatePeerSyncEntry(peerKey.String())
 
 	loginResp, err := s.prepareLoginResponse(ctx, peer, netMap, postureChecks)
 	if err != nil {

management/internals/shared/grpc/sync_fast_path.go

@@ -0,0 +1,286 @@
+package grpc
+
+import (
+	"context"
+	"net"
+	"strings"
+	"time"
+
+	log "github.com/sirupsen/logrus"
+	"golang.zx2c4.com/wireguard/wgctrl/wgtypes"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+
+	integrationsConfig "github.com/netbirdio/management-integrations/integrations/config"
+
+	"github.com/netbirdio/netbird/encryption"
+	"github.com/netbirdio/netbird/management/internals/controllers/network_map"
+	nbconfig "github.com/netbirdio/netbird/management/internals/server/config"
+	nbpeer "github.com/netbirdio/netbird/management/server/peer"
+	"github.com/netbirdio/netbird/management/server/settings"
+	"github.com/netbirdio/netbird/management/server/store"
+	nbtypes "github.com/netbirdio/netbird/management/server/types"
+	"github.com/netbirdio/netbird/shared/management/proto"
+)
+
+// peerGroupFetcher returns the group IDs a peer belongs to. It is a dependency
+// of buildFastPathResponse so tests can inject a stub without a real store.
+type peerGroupFetcher func(ctx context.Context, accountID, peerID string) ([]string, error)
+
+// peerSyncEntry records what the server last delivered to a peer on Sync so we
+// can decide whether the next Sync can skip the full network map computation.
+type peerSyncEntry struct {
+	// Serial is the NetworkMap.Serial the server last included in a full map
+	// delivered to this peer.
+	Serial uint64
+	// MetaHash is the metaHash() value of the peer metadata at the time of that
+	// delivery, used to detect a meta change on reconnect.
+	MetaHash uint64
+}
+
+// shouldSkipNetworkMap reports whether a Sync request from this peer can be
+// answered with a lightweight NetbirdConfig-only response instead of a full
+// map computation. All conditions must hold:
+//   - the peer is not Android (Android's GrpcClient.GetNetworkMap errors on nil map)
+//   - the cache holds an entry for this peer
+//   - the cached serial matches the current account serial
+//   - the cached meta hash matches the incoming meta hash
+//   - the cached serial is non-zero (guard against uninitialised entries)
+func shouldSkipNetworkMap(goOS string, hit bool, cached peerSyncEntry, currentSerial, incomingMetaHash uint64) bool {
+	if strings.EqualFold(goOS, "android") {
+		return false
+	}
+	if !hit {
+		return false
+	}
+	if cached.Serial == 0 {
+		return false
+	}
+	if cached.Serial != currentSerial {
+		return false
+	}
+	if cached.MetaHash != incomingMetaHash {
+		return false
+	}
+	return true
+}
+
+// buildFastPathResponse constructs a SyncResponse containing only NetbirdConfig
+// with fresh TURN/Relay tokens, mirroring the shape used by
+// TimeBasedAuthSecretsManager when pushing token refreshes. The response omits
+// NetworkMap, PeerConfig, Checks and RemotePeers; the client keeps its existing
+// state and only refreshes its control-plane credentials.
+func buildFastPathResponse(
+	ctx context.Context,
+	cfg *nbconfig.Config,
+	secrets SecretsManager,
+	settingsMgr settings.Manager,
+	fetchGroups peerGroupFetcher,
+	peer *nbpeer.Peer,
+) *proto.SyncResponse {
+	var turnToken *Token
+	if cfg != nil && cfg.TURNConfig != nil && cfg.TURNConfig.TimeBasedCredentials {
+		if t, err := secrets.GenerateTurnToken(); err == nil {
+			turnToken = t
+		} else {
+			log.WithContext(ctx).Warnf("fast path: generate TURN token: %v", err)
+		}
+	}
+
+	var relayToken *Token
+	if cfg != nil && cfg.Relay != nil && len(cfg.Relay.Addresses) > 0 {
+		if t, err := secrets.GenerateRelayToken(); err == nil {
+			relayToken = t
+		} else {
+			log.WithContext(ctx).Warnf("fast path: generate relay token: %v", err)
+		}
+	}
+
+	var extraSettings *nbtypes.ExtraSettings
+	if es, err := settingsMgr.GetExtraSettings(ctx, peer.AccountID); err != nil {
+		log.WithContext(ctx).Debugf("fast path: get extra settings: %v", err)
+	} else {
+		extraSettings = es
+	}
+
+	nbConfig := toNetbirdConfig(cfg, turnToken, relayToken, extraSettings)
+
+	var peerGroups []string
+	if fetchGroups != nil {
+		if ids, err := fetchGroups(ctx, peer.AccountID, peer.ID); err != nil {
+			log.WithContext(ctx).Debugf("fast path: get peer group ids: %v", err)
+		} else {
+			peerGroups = ids
+		}
+	}
+
+	nbConfig = integrationsConfig.ExtendNetBirdConfig(peer.ID, peerGroups, nbConfig, extraSettings)
+
+	return &proto.SyncResponse{NetbirdConfig: nbConfig}
+}
+
+// tryFastPathSync decides whether the current Sync can be answered with a
+// lightweight NetbirdConfig-only response. When the fast path runs, it takes
+// over the whole Sync handler (MarkPeerConnected, send, OnPeerConnected,
+// SetupRefresh, handleUpdates) and the returned took value is true.
+//
+// When took is true the caller must return the accompanying err. When took is
+// false the caller falls through to the existing slow path.
+func (s *Server) tryFastPathSync(
+	ctx context.Context,
+	reqStart, syncStart time.Time,
+	accountID string,
+	peerKey wgtypes.Key,
+	peerMeta nbpeer.PeerSystemMeta,
+	realIP net.IP,
+	peerMetaHash uint64,
+	srv proto.ManagementService_SyncServer,
+	unlock *func(),
+) (took bool, err error) {
+	if s.peerSerialCache == nil {
+		return false, nil
+	}
+	if strings.EqualFold(peerMeta.GoOS, "android") {
+		return false, nil
+	}
+
+	network, err := s.accountManager.GetStore().GetAccountNetwork(ctx, store.LockingStrengthNone, accountID)
+	if err != nil {
+		log.WithContext(ctx).Debugf("fast path: lookup account network: %v", err)
+		return false, nil
+	}
+
+	cached, hit := s.peerSerialCache.Get(peerKey.String())
+	if !shouldSkipNetworkMap(peerMeta.GoOS, hit, cached, network.CurrentSerial(), peerMetaHash) {
+		return false, nil
+	}
+
+	return true, s.runFastPathSync(ctx, reqStart, syncStart, accountID, peerKey, realIP, peerMetaHash, srv, unlock)
+}
+
+// runFastPathSync executes the fast path: mark connected, send lean response,
+// open the update channel, kick off token refresh, release the per-peer lock,
+// then block on handleUpdates until the stream is closed.
+func (s *Server) runFastPathSync(
+	ctx context.Context,
+	reqStart, syncStart time.Time,
+	accountID string,
+	peerKey wgtypes.Key,
+	realIP net.IP,
+	peerMetaHash uint64,
+	srv proto.ManagementService_SyncServer,
+	unlock *func(),
+) error {
+	if err := s.accountManager.MarkPeerConnected(ctx, peerKey.String(), true, realIP, accountID, syncStart); err != nil {
+		log.WithContext(ctx).Warnf("fast path: mark connected for peer %s: %v", peerKey.String(), err)
+	}
+
+	peer, err := s.accountManager.GetStore().GetPeerByPeerPubKey(ctx, store.LockingStrengthNone, peerKey.String())
+	if err != nil {
+		s.syncSem.Add(-1)
+		return mapError(ctx, err)
+	}
+
+	if err := s.sendFastPathResponse(ctx, peerKey, peer, srv); err != nil {
+		log.WithContext(ctx).Debugf("fast path: send response for peer %s: %v", peerKey.String(), err)
+		s.syncSem.Add(-1)
+		s.cancelPeerRoutinesWithoutLock(ctx, accountID, peer, syncStart)
+		return err
+	}
+
+	updates, err := s.networkMapController.OnPeerConnected(ctx, accountID, peer.ID)
+	if err != nil {
+		log.WithContext(ctx).Debugf("fast path: notify peer connected for %s: %v", peerKey.String(), err)
+		s.syncSem.Add(-1)
+		s.cancelPeerRoutinesWithoutLock(ctx, accountID, peer, syncStart)
+		return err
+	}
+
+	s.secretsManager.SetupRefresh(ctx, accountID, peer.ID)
+
+	if unlock != nil && *unlock != nil {
+		(*unlock)()
+		*unlock = nil
+	}
+
+	if s.appMetrics != nil {
+		s.appMetrics.GRPCMetrics().CountSyncRequestDuration(time.Since(reqStart), accountID)
+	}
+	log.WithContext(ctx).Debugf("Sync (fast path) took %s", time.Since(reqStart))
+
+	s.syncSem.Add(-1)
+
+	return s.handleUpdates(ctx, accountID, peerKey, peer, peerMetaHash, updates, srv, syncStart)
+}
+
+// sendFastPathResponse builds a NetbirdConfig-only SyncResponse, encrypts it
+// with the peer's WireGuard key and pushes it over the stream.
+func (s *Server) sendFastPathResponse(ctx context.Context, peerKey wgtypes.Key, peer *nbpeer.Peer, srv proto.ManagementService_SyncServer) error {
+	resp := buildFastPathResponse(ctx, s.config, s.secretsManager, s.settingsManager, s.fetchPeerGroups, peer)
+
+	key, err := s.secretsManager.GetWGKey()
+	if err != nil {
+		return status.Errorf(codes.Internal, "failed getting server key")
+	}
+
+	body, err := encryption.EncryptMessage(peerKey, key, resp)
+	if err != nil {
+		return status.Errorf(codes.Internal, "error encrypting fast-path sync response")
+	}
+
+	if err := srv.Send(&proto.EncryptedMessage{
+		WgPubKey: key.PublicKey().String(),
+		Body:     body,
+	}); err != nil {
+		log.WithContext(ctx).Errorf("failed sending fast-path sync response: %v", err)
+		return status.Errorf(codes.Internal, "error handling request")
+	}
+	return nil
+}
+
+// fetchPeerGroups is the dependency injected into buildFastPathResponse in
+// production. A nil accountManager store is treated as "no groups".
+func (s *Server) fetchPeerGroups(ctx context.Context, accountID, peerID string) ([]string, error) {
+	return s.accountManager.GetStore().GetPeerGroupIDs(ctx, store.LockingStrengthNone, accountID, peerID)
+}
+
+// recordPeerSyncEntry writes the serial just delivered to this peer so a
+// subsequent reconnect can take the fast path. Called after the slow path's
+// sendInitialSync has pushed a full map. A nil cache disables the fast path.
+func (s *Server) recordPeerSyncEntry(peerKey string, netMap *nbtypes.NetworkMap, peerMetaHash uint64) {
+	if s.peerSerialCache == nil {
+		return
+	}
+	if netMap == nil || netMap.Network == nil {
+		return
+	}
+	serial := netMap.Network.CurrentSerial()
+	if serial == 0 {
+		return
+	}
+	s.peerSerialCache.Set(peerKey, peerSyncEntry{Serial: serial, MetaHash: peerMetaHash})
+}
+
+// recordPeerSyncEntryFromUpdate is the sendUpdate equivalent of
+// recordPeerSyncEntry: it extracts the serial from a streamed NetworkMap update
+// so the cache stays in sync with what the peer most recently received.
+func (s *Server) recordPeerSyncEntryFromUpdate(peerKey string, update *network_map.UpdateMessage, peerMetaHash uint64) {
+	if s.peerSerialCache == nil || update == nil || update.Update == nil || update.Update.NetworkMap == nil {
+		return
+	}
+	serial := update.Update.NetworkMap.GetSerial()
+	if serial == 0 {
+		return
+	}
+	s.peerSerialCache.Set(peerKey, peerSyncEntry{Serial: serial, MetaHash: peerMetaHash})
+}
+
+// invalidatePeerSyncEntry is called after a successful Login so the next Sync
+// is guaranteed to deliver a full map, picking up whatever changed in the
+// login (SSH key rotation, approval state, user binding, etc.).
+func (s *Server) invalidatePeerSyncEntry(peerKey string) {
+	if s.peerSerialCache == nil {
+		return
+	}
+	s.peerSerialCache.Delete(peerKey)
+}

management/internals/shared/grpc/sync_fast_path_response_test.go

@@ -0,0 +1,163 @@
+package grpc
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/netbirdio/netbird/management/internals/controllers/network_map/update_channel"
+	"github.com/netbirdio/netbird/management/internals/server/config"
+	"github.com/netbirdio/netbird/management/server/groups"
+	nbpeer "github.com/netbirdio/netbird/management/server/peer"
+	"github.com/netbirdio/netbird/management/server/settings"
+	"github.com/netbirdio/netbird/management/server/types"
+	"github.com/netbirdio/netbird/util"
+)
+
+func fastPathTestPeer() *nbpeer.Peer {
+	return &nbpeer.Peer{
+		ID:        "peer-id",
+		AccountID: "account-id",
+		Key:       "pubkey",
+	}
+}
+
+func fastPathTestSecrets(t *testing.T, turn *config.TURNConfig, relay *config.Relay) *TimeBasedAuthSecretsManager {
+	t.Helper()
+	peersManager := update_channel.NewPeersUpdateManager(nil)
+	ctrl := gomock.NewController(t)
+	t.Cleanup(ctrl.Finish)
+	settingsMock := settings.NewMockManager(ctrl)
+	secrets, err := NewTimeBasedAuthSecretsManager(peersManager, turn, relay, settingsMock, groups.NewManagerMock())
+	require.NoError(t, err, "secrets manager initialisation must succeed")
+	return secrets
+}
+
+func noGroupsFetcher(context.Context, string, string) ([]string, error) {
+	return nil, nil
+}
+
+func TestBuildFastPathResponse_TimeBasedTURNAndRelay_FreshTokens(t *testing.T) {
+	ttl := util.Duration{Duration: time.Hour}
+	turnCfg := &config.TURNConfig{
+		CredentialsTTL:       ttl,
+		Secret:               "turn-secret",
+		Turns:                []*config.Host{TurnTestHost},
+		TimeBasedCredentials: true,
+	}
+	relayCfg := &config.Relay{
+		Addresses:      []string{"rel.example:443"},
+		CredentialsTTL: ttl,
+		Secret:         "relay-secret",
+	}
+	cfg := &config.Config{
+		TURNConfig: turnCfg,
+		Relay:      relayCfg,
+		Signal:     &config.Host{URI: "signal.example:443", Proto: config.HTTPS},
+		Stuns:      []*config.Host{{URI: "stun.example:3478", Proto: config.UDP}},
+	}
+
+	secrets := fastPathTestSecrets(t, turnCfg, relayCfg)
+
+	ctrl := gomock.NewController(t)
+	t.Cleanup(ctrl.Finish)
+	settingsMock := settings.NewMockManager(ctrl)
+	settingsMock.EXPECT().GetExtraSettings(gomock.Any(), "account-id").Return(&types.ExtraSettings{}, nil).AnyTimes()
+
+	resp := buildFastPathResponse(context.Background(), cfg, secrets, settingsMock, noGroupsFetcher, fastPathTestPeer())
+
+	require.NotNil(t, resp, "response must not be nil")
+	assert.Nil(t, resp.NetworkMap, "fast path must omit NetworkMap")
+	assert.Nil(t, resp.PeerConfig, "fast path must omit PeerConfig")
+	assert.Empty(t, resp.Checks, "fast path must omit posture checks")
+	assert.Empty(t, resp.RemotePeers, "fast path must omit remote peers")
+
+	require.NotNil(t, resp.NetbirdConfig, "NetbirdConfig must be present on fast path")
+	require.Len(t, resp.NetbirdConfig.Turns, 1, "time-based TURN credentials must be present")
+	assert.NotEmpty(t, resp.NetbirdConfig.Turns[0].User, "TURN user must be populated")
+	assert.NotEmpty(t, resp.NetbirdConfig.Turns[0].Password, "TURN password must be populated")
+
+	require.NotNil(t, resp.NetbirdConfig.Relay, "Relay config must be present when configured")
+	assert.NotEmpty(t, resp.NetbirdConfig.Relay.TokenPayload, "relay token payload must be populated")
+	assert.NotEmpty(t, resp.NetbirdConfig.Relay.TokenSignature, "relay token signature must be populated")
+	assert.Equal(t, []string{"rel.example:443"}, resp.NetbirdConfig.Relay.Urls, "relay URLs passthrough")
+
+	require.NotNil(t, resp.NetbirdConfig.Signal, "Signal config must be present when configured")
+	assert.Equal(t, "signal.example:443", resp.NetbirdConfig.Signal.Uri, "signal URI passthrough")
+	require.Len(t, resp.NetbirdConfig.Stuns, 1, "STUNs must be passed through")
+	assert.Equal(t, "stun.example:3478", resp.NetbirdConfig.Stuns[0].Uri, "STUN URI passthrough")
+}
+
+func TestBuildFastPathResponse_StaticTURNCredentials(t *testing.T) {
+	ttl := util.Duration{Duration: time.Hour}
+	staticHost := &config.Host{
+		URI:      "turn:static.example:3478",
+		Proto:    config.UDP,
+		Username: "preset-user",
+		Password: "preset-pass",
+	}
+	turnCfg := &config.TURNConfig{
+		CredentialsTTL:       ttl,
+		Secret:               "turn-secret",
+		Turns:                []*config.Host{staticHost},
+		TimeBasedCredentials: false,
+	}
+	cfg := &config.Config{TURNConfig: turnCfg}
+
+	// Use a relay-free secrets manager; static TURN path does not consult it.
+	secrets := fastPathTestSecrets(t, turnCfg, nil)
+
+	ctrl := gomock.NewController(t)
+	t.Cleanup(ctrl.Finish)
+	settingsMock := settings.NewMockManager(ctrl)
+	settingsMock.EXPECT().GetExtraSettings(gomock.Any(), gomock.Any()).Return(&types.ExtraSettings{}, nil).AnyTimes()
+
+	resp := buildFastPathResponse(context.Background(), cfg, secrets, settingsMock, noGroupsFetcher, fastPathTestPeer())
+
+	require.NotNil(t, resp.NetbirdConfig)
+	require.Len(t, resp.NetbirdConfig.Turns, 1, "static TURN must appear in response")
+	assert.Equal(t, "preset-user", resp.NetbirdConfig.Turns[0].User, "static user passthrough")
+	assert.Equal(t, "preset-pass", resp.NetbirdConfig.Turns[0].Password, "static password passthrough")
+	assert.Nil(t, resp.NetbirdConfig.Relay, "no Relay when Relay config is nil")
+}
+
+func TestBuildFastPathResponse_NoRelayConfigured_NoRelaySection(t *testing.T) {
+	cfg := &config.Config{}
+	secrets := fastPathTestSecrets(t, nil, nil)
+
+	ctrl := gomock.NewController(t)
+	t.Cleanup(ctrl.Finish)
+	settingsMock := settings.NewMockManager(ctrl)
+	settingsMock.EXPECT().GetExtraSettings(gomock.Any(), gomock.Any()).Return(&types.ExtraSettings{}, nil).AnyTimes()
+
+	resp := buildFastPathResponse(context.Background(), cfg, secrets, settingsMock, noGroupsFetcher, fastPathTestPeer())
+	require.NotNil(t, resp.NetbirdConfig, "NetbirdConfig must be non-nil even without relay/turn")
+	assert.Nil(t, resp.NetbirdConfig.Relay, "Relay must be absent when not configured")
+	assert.Empty(t, resp.NetbirdConfig.Turns, "Turns must be empty when not configured")
+}
+
+func TestBuildFastPathResponse_ExtraSettingsErrorStillReturnsResponse(t *testing.T) {
+	cfg := &config.Config{}
+	secrets := fastPathTestSecrets(t, nil, nil)
+
+	ctrl := gomock.NewController(t)
+	t.Cleanup(ctrl.Finish)
+	settingsMock := settings.NewMockManager(ctrl)
+	settingsMock.EXPECT().GetExtraSettings(gomock.Any(), gomock.Any()).Return(nil, assertAnError).AnyTimes()
+
+	resp := buildFastPathResponse(context.Background(), cfg, secrets, settingsMock, noGroupsFetcher, fastPathTestPeer())
+	require.NotNil(t, resp, "extra settings failure must degrade gracefully into an empty fast-path response")
+	assert.Nil(t, resp.NetworkMap, "NetworkMap still omitted on degraded path")
+}
+
+// assertAnError is a sentinel used by fast-path tests that need to simulate a
+// dependency failure without caring about the error value.
+var assertAnError = errForTests("simulated")
+
+type errForTests string
+
+func (e errForTests) Error() string { return string(e) }