diff --git a/cmd/syncthing/main.go b/cmd/syncthing/main.go
index e4f8e022..e22eb71d 100644
--- a/cmd/syncthing/main.go
+++ b/cmd/syncthing/main.go
@@ -682,17 +682,9 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 		}
 	}
 
-	// Clear out old indexes for other devices. Otherwise we'll start up and
-	// start needing a bunch of files which are nowhere to be found. This
-	// needs to be changed when we correctly do persistent indexes.
+	// Add and start folders
 	for _, folderCfg := range cfg.Folders() {
 		m.AddFolder(folderCfg)
-		for _, device := range folderCfg.DeviceIDs() {
-			if device == myID {
-				continue
-			}
-			m.Index(device, folderCfg.ID, nil)
-		}
 		m.StartFolder(folderCfg.ID)
 	}
 
diff --git a/lib/db/leveldb.go b/lib/db/leveldb.go
index 5a23ba24..c4f3abe4 100644
--- a/lib/db/leveldb.go
+++ b/lib/db/leveldb.go
@@ -11,27 +11,10 @@ import (
 	"fmt"
 
 	"github.com/syncthing/syncthing/lib/protocol"
-	"github.com/syncthing/syncthing/lib/sync"
 	"github.com/syndtr/goleveldb/leveldb"
 	"github.com/syndtr/goleveldb/leveldb/opt"
 )
 
-var (
-	clockTick int64
-	clockMut  = sync.NewMutex()
-)
-
-func clock(v int64) int64 {
-	clockMut.Lock()
-	defer clockMut.Unlock()
-	if v > clockTick {
-		clockTick = v + 1
-	} else {
-		clockTick++
-	}
-	return clockTick
-}
-
 const (
 	KeyTypeDevice = iota
 	KeyTypeGlobal
@@ -41,6 +24,7 @@ const (
 	KeyTypeVirtualMtime
 	KeyTypeFolderIdx
 	KeyTypeDeviceIdx
+	KeyTypeIndexID
 )
 
 func (l VersionList) String() string {
diff --git a/lib/db/leveldb_dbinstance.go b/lib/db/leveldb_dbinstance.go
index c3fbc3ab..687e5b16 100644
--- a/lib/db/leveldb_dbinstance.go
+++ b/lib/db/leveldb_dbinstance.go
@@ -24,7 +24,7 @@ import (
 	"github.com/syndtr/goleveldb/leveldb/util"
 )
 
-type deletionHandler func(t readWriteTransaction, folder, device, name []byte, dbi iterator.Iterator) int64
+type deletionHandler func(t readWriteTransaction, folder, device, name []byte, dbi iterator.Iterator)
 
 type Instance struct {
 	committed int64 // this must be the first attribute in the struct to ensure 64 bit alignment on 32 bit plaforms
@@ -86,7 +86,7 @@ func (db *Instance) Committed() int64 {
 	return atomic.LoadInt64(&db.committed)
 }
 
-func (db *Instance) genericReplace(folder, device []byte, fs []protocol.FileInfo, localSize, globalSize *sizeTracker, deleteFn deletionHandler) int64 {
+func (db *Instance) genericReplace(folder, device []byte, fs []protocol.FileInfo, localSize, globalSize *sizeTracker, deleteFn deletionHandler) {
 	sort.Sort(fileList(fs)) // sort list on name, same as in the database
 
 	t := db.newReadWriteTransaction()
@@ -97,7 +97,6 @@ func (db *Instance) genericReplace(folder, device []byte, fs []protocol.FileInfo
 
 	moreDb := dbi.Next()
 	fsi := 0
-	var maxLocalVer int64
 
 	isLocalDevice := bytes.Equal(device, protocol.LocalDeviceID[:])
 	for {
@@ -124,9 +123,7 @@ func (db *Instance) genericReplace(folder, device []byte, fs []protocol.FileInfo
 		case moreFs && (!moreDb || cmp == -1):
 			l.Debugln("generic replace; missing - insert")
 			// Database is missing this file. Insert it.
-			if lv := t.insertFile(folder, device, fs[fsi]); lv > maxLocalVer {
-				maxLocalVer = lv
-			}
+			t.insertFile(folder, device, fs[fsi])
 			if isLocalDevice {
 				localSize.addFile(fs[fsi])
 			}
@@ -146,9 +143,7 @@ func (db *Instance) genericReplace(folder, device []byte, fs []protocol.FileInfo
 			ef.Unmarshal(dbi.Value())
 			if !fs[fsi].Version.Equal(ef.Version) || fs[fsi].Invalid != ef.Invalid {
 				l.Debugln("generic replace; differs - insert")
-				if lv := t.insertFile(folder, device, fs[fsi]); lv > maxLocalVer {
-					maxLocalVer = lv
-				}
+				t.insertFile(folder, device, fs[fsi])
 				if isLocalDevice {
 					localSize.removeFile(ef)
 					localSize.addFile(fs[fsi])
@@ -167,9 +162,7 @@ func (db *Instance) genericReplace(folder, device []byte, fs []protocol.FileInfo
 
 		case moreDb && (!moreFs || cmp == 1):
 			l.Debugln("generic replace; exists - remove")
-			if lv := deleteFn(t, folder, device, oldName, dbi); lv > maxLocalVer {
-				maxLocalVer = lv
-			}
+			deleteFn(t, folder, device, oldName, dbi)
 			moreDb = dbi.Next()
 		}
 
@@ -177,26 +170,21 @@ func (db *Instance) genericReplace(folder, device []byte, fs []protocol.FileInfo
 		// growing too large and thus allocating unnecessarily much memory.
 		t.checkFlush()
 	}
-
-	return maxLocalVer
 }
 
-func (db *Instance) replace(folder, device []byte, fs []protocol.FileInfo, localSize, globalSize *sizeTracker) int64 {
-	// TODO: Return the remaining maxLocalVer?
-	return db.genericReplace(folder, device, fs, localSize, globalSize, func(t readWriteTransaction, folder, device, name []byte, dbi iterator.Iterator) int64 {
+func (db *Instance) replace(folder, device []byte, fs []protocol.FileInfo, localSize, globalSize *sizeTracker) {
+	db.genericReplace(folder, device, fs, localSize, globalSize, func(t readWriteTransaction, folder, device, name []byte, dbi iterator.Iterator) {
 		// Database has a file that we are missing. Remove it.
 		l.Debugf("delete; folder=%q device=%v name=%q", folder, protocol.DeviceIDFromBytes(device), name)
 		t.removeFromGlobal(folder, device, name, globalSize)
 		t.Delete(dbi.Key())
-		return 0
 	})
 }
 
-func (db *Instance) updateFiles(folder, device []byte, fs []protocol.FileInfo, localSize, globalSize *sizeTracker) int64 {
+func (db *Instance) updateFiles(folder, device []byte, fs []protocol.FileInfo, localSize, globalSize *sizeTracker) {
 	t := db.newReadWriteTransaction()
 	defer t.close()
 
-	var maxLocalVer int64
 	var fk []byte
 	isLocalDevice := bytes.Equal(device, protocol.LocalDeviceID[:])
 	for _, f := range fs {
@@ -208,9 +196,7 @@ func (db *Instance) updateFiles(folder, device []byte, fs []protocol.FileInfo, l
 				localSize.addFile(f)
 			}
 
-			if lv := t.insertFile(folder, device, f); lv > maxLocalVer {
-				maxLocalVer = lv
-			}
+			t.insertFile(folder, device, f)
 			if f.IsInvalid() {
 				t.removeFromGlobal(folder, device, name, globalSize)
 			} else {
@@ -231,9 +217,7 @@ func (db *Instance) updateFiles(folder, device []byte, fs []protocol.FileInfo, l
 				localSize.addFile(f)
 			}
 
-			if lv := t.insertFile(folder, device, f); lv > maxLocalVer {
-				maxLocalVer = lv
-			}
+			t.insertFile(folder, device, f)
 			if f.IsInvalid() {
 				t.removeFromGlobal(folder, device, name, globalSize)
 			} else {
@@ -245,8 +229,6 @@ func (db *Instance) updateFiles(folder, device []byte, fs []protocol.FileInfo, l
 		// growing too large and thus allocating unnecessarily much memory.
 		t.checkFlush()
 	}
-
-	return maxLocalVer
 }
 
 func (db *Instance) withHave(folder, device, prefix []byte, truncate bool, fn Iterator) {
@@ -699,6 +681,37 @@ func (db *Instance) globalKeyFolder(key []byte) []byte {
 	return folder
 }
 
+func (db *Instance) getIndexID(device, folder []byte) protocol.IndexID {
+	key := db.indexIDKey(device, folder)
+	cur, err := db.Get(key, nil)
+	if err != nil {
+		return 0
+	}
+
+	var id protocol.IndexID
+	if err := id.Unmarshal(cur); err != nil {
+		return 0
+	}
+
+	return id
+}
+
+func (db *Instance) setIndexID(device, folder []byte, id protocol.IndexID) {
+	key := db.indexIDKey(device, folder)
+	bs, _ := id.Marshal() // marshalling can't fail
+	if err := db.Put(key, bs, nil); err != nil {
+		panic("storing index ID: " + err.Error())
+	}
+}
+
+func (db *Instance) indexIDKey(device, folder []byte) []byte {
+	k := make([]byte, keyPrefixLen+keyDeviceLen+keyFolderLen)
+	k[0] = KeyTypeIndexID
+	binary.BigEndian.PutUint32(k[keyPrefixLen:], db.deviceIdx.ID(device))
+	binary.BigEndian.PutUint32(k[keyPrefixLen+keyDeviceLen:], db.folderIdx.ID(folder))
+	return k
+}
+
 func unmarshalTrunc(bs []byte, truncate bool) (FileIntf, error) {
 	if truncate {
 		var tf FileInfoTruncated
diff --git a/lib/db/leveldb_transactions.go b/lib/db/leveldb_transactions.go
index 409d812f..8fa50974 100644
--- a/lib/db/leveldb_transactions.go
+++ b/lib/db/leveldb_transactions.go
@@ -74,18 +74,12 @@ func (t readWriteTransaction) flush() {
 	atomic.AddInt64(&t.db.committed, int64(t.Batch.Len()))
 }
 
-func (t readWriteTransaction) insertFile(folder, device []byte, file protocol.FileInfo) int64 {
+func (t readWriteTransaction) insertFile(folder, device []byte, file protocol.FileInfo) {
 	l.Debugf("insert; folder=%q device=%v %v", folder, protocol.DeviceIDFromBytes(device), file)
 
-	if file.LocalVersion == 0 {
-		file.LocalVersion = clock(0)
-	}
-
 	name := []byte(file.Name)
 	nk := t.db.deviceKey(folder, device, name)
 	t.Put(nk, mustMarshal(&file))
-
-	return file.LocalVersion
 }
 
 // updateGlobal adds this device+version to the version list for the given
diff --git a/lib/db/set.go b/lib/db/set.go
index 79d30f45..df72e576 100644
--- a/lib/db/set.go
+++ b/lib/db/set.go
@@ -14,6 +14,7 @@ package db
 
 import (
 	stdsync "sync"
+	"sync/atomic"
 
 	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/protocol"
@@ -21,13 +22,15 @@ import (
 )
 
 type FileSet struct {
-	localVersion map[protocol.DeviceID]int64
-	mutex        sync.Mutex
+	localVersion int64 // Our local version counter
 	folder       string
 	db           *Instance
 	blockmap     *BlockMap
 	localSize    sizeTracker
 	globalSize   sizeTracker
+
+	remoteLocalVersion map[protocol.DeviceID]int64 // Highest seen local versions for other devices
+	rlvMutex           sync.Mutex                  // protects remoteLocalVersion
 }
 
 // FileIntf is the set of methods implemented by both protocol.FileInfo and
@@ -95,11 +98,11 @@ func (s *sizeTracker) Size() (files, deleted int, bytes int64) {
 
 func NewFileSet(folder string, db *Instance) *FileSet {
 	var s = FileSet{
-		localVersion: make(map[protocol.DeviceID]int64),
-		folder:       folder,
-		db:           db,
-		blockmap:     NewBlockMap(db, db.folderIdx.ID([]byte(folder))),
-		mutex:        sync.NewMutex(),
+		remoteLocalVersion: make(map[protocol.DeviceID]int64),
+		folder:             folder,
+		db:                 db,
+		blockmap:           NewBlockMap(db, db.folderIdx.ID([]byte(folder))),
+		rlvMutex:           sync.NewMutex(),
 	}
 
 	s.db.checkGlobals([]byte(folder), &s.globalSize)
@@ -107,16 +110,17 @@ func NewFileSet(folder string, db *Instance) *FileSet {
 	var deviceID protocol.DeviceID
 	s.db.withAllFolderTruncated([]byte(folder), func(device []byte, f FileInfoTruncated) bool {
 		copy(deviceID[:], device)
-		if f.LocalVersion > s.localVersion[deviceID] {
-			s.localVersion[deviceID] = f.LocalVersion
-		}
 		if deviceID == protocol.LocalDeviceID {
+			if f.LocalVersion > s.localVersion {
+				s.localVersion = f.LocalVersion
+			}
 			s.localSize.addFile(f)
+		} else if f.LocalVersion > s.remoteLocalVersion[deviceID] {
+			s.remoteLocalVersion[deviceID] = f.LocalVersion
 		}
 		return true
 	})
 	l.Debugf("loaded localVersion for %q: %#v", folder, s.localVersion)
-	clock(s.localVersion[protocol.LocalDeviceID])
 
 	return &s
 }
@@ -124,13 +128,23 @@ func NewFileSet(folder string, db *Instance) *FileSet {
 func (s *FileSet) Replace(device protocol.DeviceID, fs []protocol.FileInfo) {
 	l.Debugf("%s Replace(%v, [%d])", s.folder, device, len(fs))
 	normalizeFilenames(fs)
-	s.mutex.Lock()
-	defer s.mutex.Unlock()
-	s.localVersion[device] = s.db.replace([]byte(s.folder), device[:], fs, &s.localSize, &s.globalSize)
-	if len(fs) == 0 {
-		// Reset the local version if all files were removed.
-		s.localVersion[device] = 0
+	if device == protocol.LocalDeviceID {
+		if len(fs) == 0 {
+			s.localVersion = 0
+		} else {
+			// Always overwrite LocalVersion on updated files to ensure
+			// correct ordering. The caller is supposed to leave it set to
+			// zero anyhow.
+			for i := range fs {
+				fs[i].LocalVersion = atomic.AddInt64(&s.localVersion, 1)
+			}
+		}
+	} else {
+		s.rlvMutex.Lock()
+		s.remoteLocalVersion[device] = maxLocalVersion(fs)
+		s.rlvMutex.Unlock()
 	}
+	s.db.replace([]byte(s.folder), device[:], fs, &s.localSize, &s.globalSize)
 	if device == protocol.LocalDeviceID {
 		s.blockmap.Drop()
 		s.blockmap.Add(fs)
@@ -140,12 +154,11 @@ func (s *FileSet) Replace(device protocol.DeviceID, fs []protocol.FileInfo) {
 func (s *FileSet) Update(device protocol.DeviceID, fs []protocol.FileInfo) {
 	l.Debugf("%s Update(%v, [%d])", s.folder, device, len(fs))
 	normalizeFilenames(fs)
-	s.mutex.Lock()
-	defer s.mutex.Unlock()
 	if device == protocol.LocalDeviceID {
 		discards := make([]protocol.FileInfo, 0, len(fs))
 		updates := make([]protocol.FileInfo, 0, len(fs))
-		for _, newFile := range fs {
+		for i, newFile := range fs {
+			fs[i].LocalVersion = atomic.AddInt64(&s.localVersion, 1)
 			existingFile, ok := s.db.getFile([]byte(s.folder), device[:], []byte(newFile.Name))
 			if !ok || !existingFile.Version.Equal(newFile.Version) {
 				discards = append(discards, existingFile)
@@ -154,10 +167,12 @@ func (s *FileSet) Update(device protocol.DeviceID, fs []protocol.FileInfo) {
 		}
 		s.blockmap.Discard(discards)
 		s.blockmap.Update(updates)
+	} else {
+		s.rlvMutex.Lock()
+		s.remoteLocalVersion[device] = maxLocalVersion(fs)
+		s.rlvMutex.Unlock()
 	}
-	if lv := s.db.updateFiles([]byte(s.folder), device[:], fs, &s.localSize, &s.globalSize); lv > s.localVersion[device] {
-		s.localVersion[device] = lv
-	}
+	s.db.updateFiles([]byte(s.folder), device[:], fs, &s.localSize, &s.globalSize)
 }
 
 func (s *FileSet) WithNeed(device protocol.DeviceID, fn Iterator) {
@@ -230,9 +245,13 @@ func (s *FileSet) Availability(file string) []protocol.DeviceID {
 }
 
 func (s *FileSet) LocalVersion(device protocol.DeviceID) int64 {
-	s.mutex.Lock()
-	defer s.mutex.Unlock()
-	return s.localVersion[device]
+	if device == protocol.LocalDeviceID {
+		return atomic.LoadInt64(&s.localVersion)
+	}
+
+	s.rlvMutex.Lock()
+	defer s.rlvMutex.Unlock()
+	return s.remoteLocalVersion[device]
 }
 
 func (s *FileSet) LocalSize() (files, deleted int, bytes int64) {
@@ -243,6 +262,37 @@ func (s *FileSet) GlobalSize() (files, deleted int, bytes int64) {
 	return s.globalSize.Size()
 }
 
+func (s *FileSet) IndexID(device protocol.DeviceID) protocol.IndexID {
+	id := s.db.getIndexID(device[:], []byte(s.folder))
+	if id == 0 && device == protocol.LocalDeviceID {
+		// No index ID set yet. We create one now.
+		id = protocol.NewIndexID()
+		s.db.setIndexID(device[:], []byte(s.folder), id)
+	}
+	return id
+}
+
+func (s *FileSet) SetIndexID(device protocol.DeviceID, id protocol.IndexID) {
+	if device == protocol.LocalDeviceID {
+		panic("do not explicitly set index ID for local device")
+	}
+	s.db.setIndexID(device[:], []byte(s.folder), id)
+}
+
+// maxLocalVersion returns the highest of the LocalVersion numbers found in
+// the given slice of FileInfos. This should really be the LocalVersion of
+// the last item, but Syncthing v0.14.0 and other implementations may not
+// implement update sorting....
+func maxLocalVersion(fs []protocol.FileInfo) int64 {
+	var max int64
+	for _, f := range fs {
+		if f.LocalVersion > max {
+			max = f.LocalVersion
+		}
+	}
+	return max
+}
+
 // DropFolder clears out all information related to the given folder from the
 // database.
 func DropFolder(db *Instance, folder string) {
diff --git a/lib/db/set_test.go b/lib/db/set_test.go
index 32bedad8..48d6525a 100644
--- a/lib/db/set_test.go
+++ b/lib/db/set_test.go
@@ -100,11 +100,11 @@ func TestGlobalSet(t *testing.T) {
 	m := db.NewFileSet("test", ldb)
 
 	local0 := fileList{
-		protocol.FileInfo{Name: "a", Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(1)},
-		protocol.FileInfo{Name: "b", Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(2)},
-		protocol.FileInfo{Name: "c", Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(3)},
-		protocol.FileInfo{Name: "d", Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(4)},
-		protocol.FileInfo{Name: "z", Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(8)},
+		protocol.FileInfo{Name: "a", LocalVersion: 1, Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(1)},
+		protocol.FileInfo{Name: "b", LocalVersion: 2, Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(2)},
+		protocol.FileInfo{Name: "c", LocalVersion: 3, Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(3)},
+		protocol.FileInfo{Name: "d", LocalVersion: 4, Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(4)},
+		protocol.FileInfo{Name: "z", LocalVersion: 5, Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(8)},
 	}
 	local1 := fileList{
 		protocol.FileInfo{Name: "a", Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1000}}}, Blocks: genBlocks(1)},
@@ -687,3 +687,35 @@ func BenchmarkUpdateOneFile(b *testing.B) {
 
 	b.ReportAllocs()
 }
+
+func TestIndexID(t *testing.T) {
+	ldb := db.OpenMemory()
+
+	s := db.NewFileSet("test", ldb)
+
+	// The Index ID for some random device is zero by default.
+	id := s.IndexID(remoteDevice0)
+	if id != 0 {
+		t.Errorf("index ID for remote device should default to zero, not %d", id)
+	}
+
+	// The Index ID for someone else should be settable
+	s.SetIndexID(remoteDevice0, 42)
+	id = s.IndexID(remoteDevice0)
+	if id != 42 {
+		t.Errorf("index ID for remote device should be remembered; got %d, expected %d", id, 42)
+	}
+
+	// Our own index ID should be generated randomly.
+	id = s.IndexID(protocol.LocalDeviceID)
+	if id == 0 {
+		t.Errorf("index ID for local device should be random, not zero")
+	}
+	t.Logf("random index ID is 0x%016x", id)
+
+	// But of course always the same after that.
+	again := s.IndexID(protocol.LocalDeviceID)
+	if again != id {
+		t.Errorf("index ID changed; %d != %d", again, id)
+	}
+}
diff --git a/lib/model/model.go b/lib/model/model.go
index 56a50b84..3d1151ed 100644
--- a/lib/model/model.go
+++ b/lib/model/model.go
@@ -8,6 +8,7 @@ package model
 
 import (
 	"bufio"
+	"bytes"
 	"crypto/tls"
 	"encoding/json"
 	"errors"
@@ -647,6 +648,10 @@ func (m *Model) ClusterConfig(deviceID protocol.DeviceID, cm protocol.ClusterCon
 
 	tempIndexFolders := make([]string, 0, len(cm.Folders))
 
+	m.pmut.RLock()
+	conn := m.conn[deviceID]
+	m.pmut.RUnlock()
+
 	m.fmut.Lock()
 	for _, folder := range cm.Folders {
 		if !m.folderSharedWithUnlocked(folder.ID, deviceID) {
@@ -661,6 +666,71 @@ func (m *Model) ClusterConfig(deviceID protocol.DeviceID, cm protocol.ClusterCon
 		if !folder.DisableTempIndexes {
 			tempIndexFolders = append(tempIndexFolders, folder.ID)
 		}
+
+		fs := m.folderFiles[folder.ID]
+		myIndexID := fs.IndexID(protocol.LocalDeviceID)
+		myLocalVersion := fs.LocalVersion(protocol.LocalDeviceID)
+		var startLocalVersion int64
+
+		for _, dev := range folder.Devices {
+			if bytes.Equal(dev.ID, m.id[:]) {
+				// This is the other side's description of what it knows
+				// about us. Lets check to see if we can start sending index
+				// updates directly or need to send the index from start...
+
+				if dev.IndexID == myIndexID {
+					// They say they've seen our index ID before, so we can
+					// send a delta update only.
+
+					if dev.MaxLocalVersion > myLocalVersion {
+						// Safety check. They claim to have more or newer
+						// index data than we have - either we have lost
+						// index data, or reset the index without resetting
+						// the IndexID, or something else weird has
+						// happened. We send a full index to reset the
+						// situation.
+						l.Infof("Device %v folder %q is delta index compatible, but seems out of sync with reality", deviceID, folder.ID)
+						startLocalVersion = 0
+						continue
+					}
+
+					l.Infof("Device %v folder %q is delta index compatible (mlv=%d)", deviceID, folder.ID, dev.MaxLocalVersion)
+					startLocalVersion = dev.MaxLocalVersion
+				} else if dev.IndexID != 0 {
+					// They say they've seen an index ID from us, but it's
+					// not the right one. Either they are confused or we
+					// must have reset our database since last talking to
+					// them. We'll start with a full index transfer.
+					l.Infof("Device %v folder %q has mismatching index ID for us (%v != %v)", deviceID, folder.ID, dev.IndexID, myIndexID)
+					startLocalVersion = 0
+				}
+			} else if bytes.Equal(dev.ID, deviceID[:]) && dev.IndexID != 0 {
+				// This is the other side's description of themselves. We
+				// check to see that it matches the IndexID we have on file,
+				// otherwise we drop our old index data and expect to get a
+				// completely new set.
+
+				theirIndexID := fs.IndexID(deviceID)
+				if dev.IndexID == 0 {
+					// They're not announcing an index ID. This means they
+					// do not support delta indexes and we should clear any
+					// information we have from them before accepting their
+					// index, which will presumably be a full index.
+					fs.Replace(deviceID, nil)
+				} else if dev.IndexID != theirIndexID {
+					// The index ID we have on file is not what they're
+					// announcing. They must have reset their database and
+					// will probably send us a full index. We drop any
+					// information we have and remember this new index ID
+					// instead.
+					l.Infof("Device %v folder %q has a new index ID (%v)", deviceID, folder.ID, dev.IndexID)
+					fs.Replace(deviceID, nil)
+					fs.SetIndexID(deviceID, dev.IndexID)
+				}
+			}
+		}
+
+		go sendIndexes(conn, folder.ID, fs, m.folderIgnores[folder.ID], startLocalVersion)
 	}
 	m.fmut.Unlock()
 
@@ -763,12 +833,6 @@ func (m *Model) Close(device protocol.DeviceID, err error) {
 	})
 
 	m.pmut.Lock()
-	m.fmut.RLock()
-	for _, folder := range m.deviceFolders[device] {
-		m.folderFiles[folder].Replace(device, nil)
-	}
-	m.fmut.RUnlock()
-
 	conn, ok := m.conn[device]
 	if ok {
 		m.progressEmitter.temporaryIndexUnsubscribe(conn)
@@ -1044,13 +1108,6 @@ func (m *Model) AddConnection(conn connections.Connection, hello protocol.HelloR
 
 	cm := m.generateClusterConfig(deviceID)
 	conn.ClusterConfig(cm)
-
-	m.fmut.RLock()
-	for _, folder := range m.deviceFolders[deviceID] {
-		fs := m.folderFiles[folder]
-		go sendIndexes(conn, folder, fs, m.folderIgnores[folder])
-	}
-	m.fmut.RUnlock()
 	m.pmut.Unlock()
 
 	device, ok := m.cfg.Devices()[deviceID]
@@ -1146,15 +1203,15 @@ func (m *Model) receivedFile(folder string, file protocol.FileInfo) {
 	m.folderStatRef(folder).ReceivedFile(file.Name, file.IsDeleted())
 }
 
-func sendIndexes(conn protocol.Connection, folder string, fs *db.FileSet, ignores *ignore.Matcher) {
+func sendIndexes(conn protocol.Connection, folder string, fs *db.FileSet, ignores *ignore.Matcher, startLocalVersion int64) {
 	deviceID := conn.ID()
 	name := conn.Name()
 	var err error
 
-	l.Debugf("sendIndexes for %s-%s/%q starting", deviceID, name, folder)
+	l.Debugf("sendIndexes for %s-%s/%q starting (slv=%d)", deviceID, name, folder, startLocalVersion)
 	defer l.Debugf("sendIndexes for %s-%s/%q exiting: %v", deviceID, name, folder, err)
 
-	minLocalVer, err := sendIndexTo(true, 0, conn, folder, fs, ignores)
+	minLocalVer, err := sendIndexTo(startLocalVersion, conn, folder, fs, ignores)
 
 	// Subscribe to LocalIndexUpdated (we have new information to send) and
 	// DeviceDisconnected (it might be us who disconnected, so we should
@@ -1177,7 +1234,7 @@ func sendIndexes(conn protocol.Connection, folder string, fs *db.FileSet, ignore
 			continue
 		}
 
-		minLocalVer, err = sendIndexTo(false, minLocalVer, conn, folder, fs, ignores)
+		minLocalVer, err = sendIndexTo(minLocalVer, conn, folder, fs, ignores)
 
 		// Wait a short amount of time before entering the next loop. If there
 		// are continuous changes happening to the local index, this gives us
@@ -1186,12 +1243,13 @@ func sendIndexes(conn protocol.Connection, folder string, fs *db.FileSet, ignore
 	}
 }
 
-func sendIndexTo(initial bool, minLocalVer int64, conn protocol.Connection, folder string, fs *db.FileSet, ignores *ignore.Matcher) (int64, error) {
+func sendIndexTo(minLocalVer int64, conn protocol.Connection, folder string, fs *db.FileSet, ignores *ignore.Matcher) (int64, error) {
 	deviceID := conn.ID()
 	name := conn.Name()
 	batch := make([]protocol.FileInfo, 0, indexBatchSize)
 	currentBatchSize := 0
-	maxLocalVer := int64(0)
+	initial := minLocalVer == 0
+	maxLocalVer := minLocalVer
 	var err error
 
 	sorter := NewIndexSorter()
@@ -1340,7 +1398,7 @@ func (m *Model) requestGlobal(deviceID protocol.DeviceID, folder, name string, o
 		return nil, fmt.Errorf("requestGlobal: no such device: %s", deviceID)
 	}
 
-	l.Debugf("%v REQ(out): %s: %q / %q o=%d s=%d h=%x ft=%t op=%s", m, deviceID, folder, name, offset, size, hash, fromTemporary)
+	l.Debugf("%v REQ(out): %s: %q / %q o=%d s=%d h=%x ft=%t", m, deviceID, folder, name, offset, size, hash, fromTemporary)
 
 	return nc.Request(folder, name, offset, size, hash, fromTemporary)
 }
@@ -1660,6 +1718,8 @@ func (m *Model) generateClusterConfig(device protocol.DeviceID) protocol.Cluster
 	m.fmut.RLock()
 	for _, folder := range m.deviceFolders[device] {
 		folderCfg := m.cfg.Folders()[folder]
+		fs := m.folderFiles[folder]
+
 		protocolFolder := protocol.Folder{
 			ID:                 folder,
 			Label:              folderCfg.Label,
@@ -1676,13 +1736,26 @@ func (m *Model) generateClusterConfig(device protocol.DeviceID) protocol.Cluster
 			// TODO: Set read only bit when relevant, and when we have per device
 			// access controls.
 			deviceCfg := m.cfg.Devices()[device]
+
+			var indexID protocol.IndexID
+			var maxLocalVersion int64
+			if device == m.id {
+				indexID = fs.IndexID(protocol.LocalDeviceID)
+				maxLocalVersion = fs.LocalVersion(protocol.LocalDeviceID)
+			} else {
+				indexID = fs.IndexID(device)
+				maxLocalVersion = fs.LocalVersion(device)
+			}
+
 			protocolDevice := protocol.Device{
-				ID:          device[:],
-				Name:        deviceCfg.Name,
-				Addresses:   deviceCfg.Addresses,
-				Compression: deviceCfg.Compression,
-				CertName:    deviceCfg.CertName,
-				Introducer:  deviceCfg.Introducer,
+				ID:              device[:],
+				Name:            deviceCfg.Name,
+				Addresses:       deviceCfg.Addresses,
+				Compression:     deviceCfg.Compression,
+				CertName:        deviceCfg.CertName,
+				Introducer:      deviceCfg.Introducer,
+				IndexID:         indexID,
+				MaxLocalVersion: maxLocalVersion,
 			}
 
 			protocolFolder.Devices = append(protocolFolder.Devices, protocolDevice)
diff --git a/lib/model/rwfolder.go b/lib/model/rwfolder.go
index ab9f3854..85d42ba2 100644
--- a/lib/model/rwfolder.go
+++ b/lib/model/rwfolder.go
@@ -433,12 +433,21 @@ func (f *rwFolder) pullerIteration(ignores *ignore.Matcher) int {
 		l.Debugln(f, "handling", file.Name)
 
 		if !handleFile(file) {
-			// A new or changed file or symlink. This is the only case where we
-			// do stuff concurrently in the background
-			f.queue.Push(file.Name, file.Size, file.Modified)
+			// A new or changed file or symlink. This is the only case where
+			// we do stuff concurrently in the background. We only queue
+			// files where we are connected to at least one device that has
+			// the file.
+
+			devices := folderFiles.Availability(file.Name)
+			for _, dev := range devices {
+				if f.model.ConnectedTo(dev) {
+					f.queue.Push(file.Name, file.Size, file.Modified)
+					changed++
+					break
+				}
+			}
 		}
 
-		changed++
 		return true
 	})
 
diff --git a/lib/protocol/bep.pb.go b/lib/protocol/bep.pb.go
index 5e8b5559..2aa07217 100644
--- a/lib/protocol/bep.pb.go
+++ b/lib/protocol/bep.pb.go
@@ -262,6 +262,7 @@ type Device struct {
 	CertName        string      `protobuf:"bytes,5,opt,name=cert_name,json=certName,proto3" json:"cert_name,omitempty"`
 	MaxLocalVersion int64       `protobuf:"varint,6,opt,name=max_local_version,json=maxLocalVersion,proto3" json:"max_local_version,omitempty"`
 	Introducer      bool        `protobuf:"varint,7,opt,name=introducer,proto3" json:"introducer,omitempty"`
+	IndexID         IndexID     `protobuf:"varint,8,opt,name=index_id,json=indexId,proto3,customtype=IndexID" json:"index_id"`
 }
 
 func (m *Device) Reset()                    { *m = Device{} }
@@ -672,6 +673,11 @@ func (m *Device) MarshalTo(data []byte) (int, error) {
 		}
 		i++
 	}
+	if m.IndexID != 0 {
+		data[i] = 0x40
+		i++
+		i = encodeVarintBep(data, i, uint64(m.IndexID))
+	}
 	return i, nil
 }
 
@@ -1288,6 +1294,9 @@ func (m *Device) ProtoSize() (n int) {
 	if m.Introducer {
 		n += 2
 	}
+	if m.IndexID != 0 {
+		n += 1 + sovBep(uint64(m.IndexID))
+	}
 	return n
 }
 
@@ -2246,6 +2255,25 @@ func (m *Device) Unmarshal(data []byte) error {
 				}
 			}
 			m.Introducer = bool(v != 0)
+		case 8:
+			if wireType != 0 {
+				return fmt.Errorf("proto: wrong wireType = %d for field IndexID", wireType)
+			}
+			m.IndexID = 0
+			for shift := uint(0); ; shift += 7 {
+				if shift >= 64 {
+					return ErrIntOverflowBep
+				}
+				if iNdEx >= l {
+					return io.ErrUnexpectedEOF
+				}
+				b := data[iNdEx]
+				iNdEx++
+				m.IndexID |= (IndexID(b) & 0x7F) << shift
+				if b < 0x80 {
+					break
+				}
+			}
 		default:
 			iNdEx = preIndex
 			skippy, err := skipBep(data[iNdEx:])
@@ -3898,102 +3926,104 @@ var (
 )
 
 var fileDescriptorBep = []byte{
-	// 1543 bytes of a gzipped FileDescriptorProto
-	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x09, 0x6e, 0x88, 0x02, 0xff, 0xac, 0x56, 0x41, 0x6f, 0x1a, 0xc7,
-	0x17, 0x37, 0xb0, 0x2c, 0x30, 0xc6, 0x0e, 0x9e, 0x38, 0x0e, 0xff, 0x8d, 0xff, 0xb6, 0xbb, 0x49,
-	0x54, 0x17, 0x35, 0x4e, 0x9b, 0x54, 0x8d, 0x54, 0xa9, 0x95, 0x30, 0xac, 0x1d, 0x14, 0xbc, 0x90,
-	0x05, 0x9c, 0xa6, 0x87, 0xa2, 0x85, 0x1d, 0xf0, 0x2a, 0xcb, 0x0e, 0xdd, 0x5d, 0x92, 0xb8, 0x5f,
-	0xa0, 0x52, 0xfb, 0x05, 0x7a, 0xa9, 0x94, 0x6b, 0xef, 0xfd, 0x10, 0x39, 0x46, 0x39, 0xf6, 0x10,
-	0xb5, 0xe9, 0xa5, 0x1f, 0xa0, 0xbd, 0xf7, 0xcd, 0xcc, 0x2e, 0x2c, 0xc6, 0xae, 0x72, 0xe8, 0x01,
-	0x31, 0xf3, 0xde, 0x6f, 0xde, 0xcc, 0xfc, 0xde, 0xef, 0xbd, 0x59, 0x94, 0xeb, 0x91, 0xf1, 0xde,
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-	0x8f, 0xe4, 0xfb, 0xc4, 0xb4, 0x88, 0x87, 0x3f, 0x40, 0x52, 0x70, 0x3a, 0x16, 0x7b, 0xad, 0xde,
-	0xb9, 0xb2, 0x17, 0xdd, 0x61, 0xef, 0x88, 0xf8, 0xbe, 0x39, 0x24, 0x6d, 0x70, 0x1a, 0x1c, 0x82,
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-	0x33, 0x8c, 0x11, 0x5f, 0xa0, 0x96, 0xd1, 0x4a, 0xc5, 0x99, 0xf8, 0x01, 0xf1, 0x2a, 0xd4, 0x1d,
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-	0xc2, 0x2c, 0xd8, 0x01, 0x77, 0xec, 0x4b, 0x2f, 0xdf, 0x6c, 0x2f, 0x19, 0x11, 0x4c, 0xfd, 0x21,
-	0x89, 0x64, 0xe1, 0xc1, 0x1b, 0x28, 0x69, 0x5b, 0x82, 0xa2, 0x7d, 0xf9, 0xed, 0x9b, 0xed, 0x64,
-	0xad, 0x6a, 0x80, 0x05, 0xaf, 0xa3, 0xb4, 0x63, 0xf6, 0x88, 0x13, 0x92, 0x23, 0x26, 0xf8, 0x1a,
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+	0xe1, 0x5d, 0xe2, 0xa0, 0x9a, 0xe5, 0x72, 0x71, 0xb3, 0x4f, 0xbf, 0x42, 0xfb, 0xf1, 0x41, 0xa3,
+	0xae, 0x3d, 0xe8, 0xf1, 0x88, 0x49, 0xf9, 0x1a, 0x80, 0xaf, 0xc6, 0xc1, 0xed, 0xe3, 0xb1, 0x6d,
+	0x39, 0x4f, 0x78, 0xe0, 0x7b, 0x68, 0x25, 0x84, 0x47, 0x1b, 0xa4, 0xe4, 0x2d, 0x58, 0xb3, 0x7e,
+	0xce, 0x9a, 0x68, 0x9f, 0xbb, 0xe8, 0x52, 0xb8, 0xb0, 0xab, 0x3d, 0xd0, 0x40, 0x16, 0xa0, 0x9c,
+	0x0d, 0x58, 0x26, 0x9f, 0xb3, 0xac, 0xeb, 0x3c, 0x71, 0x40, 0x14, 0xe5, 0x5f, 0x12, 0x28, 0x3f,
+	0xeb, 0x50, 0x8c, 0x67, 0xad, 0xd9, 0x53, 0x75, 0xbd, 0xa9, 0x87, 0x17, 0x9f, 0x39, 0x35, 0xca,
+	0x87, 0xf8, 0x3d, 0x94, 0xdd, 0x57, 0x35, 0x55, 0xaf, 0x57, 0xc3, 0x7a, 0x98, 0x41, 0xf6, 0x89,
+	0x43, 0x5c, 0x6b, 0x00, 0x1f, 0xfc, 0x05, 0x08, 0xd3, 0xee, 0x56, 0xef, 0x87, 0x37, 0xe6, 0x02,
+	0x8e, 0x85, 0x6a, 0x4f, 0x07, 0x47, 0xfc, 0xb6, 0x65, 0x56, 0x3a, 0x87, 0x95, 0x46, 0xbd, 0x26,
+	0xa0, 0x29, 0xb9, 0x04, 0xd0, 0xd5, 0x19, 0xb4, 0x2e, 0x9e, 0x77, 0x86, 0x2d, 0x9b, 0x68, 0xe3,
+	0xbf, 0x7b, 0x11, 0x7c, 0x76, 0x64, 0x2a, 0xad, 0x96, 0xaa, 0xd5, 0xc2, 0xd3, 0x47, 0xbe, 0xca,
+	0x64, 0x42, 0x1c, 0x93, 0x21, 0xf6, 0x9a, 0xfa, 0xbe, 0xda, 0x09, 0x0f, 0x1f, 0x21, 0xf6, 0xa8,
+	0x3b, 0x22, 0xfe, 0xee, 0xfa, 0xcb, 0x3f, 0x37, 0x16, 0x5e, 0xc1, 0xef, 0xe5, 0xc9, 0x46, 0xe2,
+	0x15, 0xfc, 0xfe, 0x38, 0xd9, 0x58, 0xf8, 0x1b, 0xfe, 0x5f, 0xfc, 0xb5, 0x91, 0xe8, 0x67, 0x78,
+	0xef, 0xba, 0xfb, 0x6f, 0x00, 0x00, 0x00, 0xff, 0xff, 0x5f, 0x7a, 0x46, 0xf6, 0x94, 0x0d, 0x00,
+	0x00,
 }
diff --git a/lib/protocol/bep.proto b/lib/protocol/bep.proto
index ecc9397d..74664852 100644
--- a/lib/protocol/bep.proto
+++ b/lib/protocol/bep.proto
@@ -70,6 +70,7 @@ message Device {
     string          cert_name         = 5;
     int64           max_local_version = 6;
     bool            introducer        = 7;
+    uint64          index_id          = 8 [(gogoproto.customname) = "IndexID", (gogoproto.customtype) = "IndexID", (gogoproto.nullable) = false];
 }
 
 enum Compression {
diff --git a/lib/protocol/bep_extensions.go b/lib/protocol/bep_extensions.go
index 3c92c947..def2a253 100644
--- a/lib/protocol/bep_extensions.go
+++ b/lib/protocol/bep_extensions.go
@@ -8,7 +8,11 @@ package protocol
 import (
 	"bytes"
 	"crypto/sha256"
+	"encoding/binary"
+	"errors"
 	"fmt"
+
+	"github.com/syncthing/syncthing/lib/rand"
 )
 
 var (
@@ -21,8 +25,8 @@ func (m Hello) Magic() uint32 {
 }
 
 func (f FileInfo) String() string {
-	return fmt.Sprintf("File{Name:%q, Permissions:0%o, Modified:%d, Version:%v, Length:%d, Deleted:%v, Invalid:%v, NoPermissions:%v, Blocks:%v}",
-		f.Name, f.Permissions, f.Modified, f.Version, f.Size, f.Deleted, f.Invalid, f.NoPermissions, f.Blocks)
+	return fmt.Sprintf("File{Name:%q, Type:%v, LocalVersion:%d, Permissions:0%o, Modified:%d, Version:%v, Length:%d, Deleted:%v, Invalid:%v, NoPermissions:%v, Blocks:%v}",
+		f.Name, f.Type, f.LocalVersion, f.Permissions, f.Modified, f.Version, f.Size, f.Deleted, f.Invalid, f.NoPermissions, f.Blocks)
 }
 
 func (f FileInfo) IsDeleted() bool {
@@ -94,3 +98,27 @@ func (b BlockInfo) String() string {
 func (b BlockInfo) IsEmpty() bool {
 	return b.Size == BlockSize && bytes.Equal(b.Hash, sha256OfEmptyBlock[:])
 }
+
+type IndexID uint64
+
+func (i IndexID) String() string {
+	return fmt.Sprintf("0x%16X", uint64(i))
+}
+
+func (i IndexID) Marshal() ([]byte, error) {
+	bs := make([]byte, 8)
+	binary.BigEndian.PutUint64(bs, uint64(i))
+	return bs, nil
+}
+
+func (i *IndexID) Unmarshal(bs []byte) error {
+	if len(bs) != 8 {
+		return errors.New("incorrect IndexID length")
+	}
+	*i = IndexID(binary.BigEndian.Uint64(bs))
+	return nil
+}
+
+func NewIndexID() IndexID {
+	return IndexID(rand.Int64())
+}