diff --git a/common_test.go b/common_test.go
index f67fb4812..f46b6a8da 100644
--- a/common_test.go
+++ b/common_test.go
@@ -13,6 +13,9 @@ type TestModel struct {
 	name     string
 	offset   int64
 	size     int
+	hash     []byte
+	flags    uint32
+	options  []Option
 	closedCh chan bool
 }
 
@@ -22,17 +25,20 @@ func newTestModel() *TestModel {
 	}
 }
 
-func (t *TestModel) Index(deviceID DeviceID, folder string, files []FileInfo) {
+func (t *TestModel) Index(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option) {
 }
 
-func (t *TestModel) IndexUpdate(deviceID DeviceID, folder string, files []FileInfo) {
+func (t *TestModel) IndexUpdate(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option) {
 }
 
-func (t *TestModel) Request(deviceID DeviceID, folder, name string, offset int64, size int) ([]byte, error) {
+func (t *TestModel) Request(deviceID DeviceID, folder, name string, offset int64, size int, hash []byte, flags uint32, options []Option) ([]byte, error) {
 	t.folder = folder
 	t.name = name
 	t.offset = offset
 	t.size = size
+	t.hash = hash
+	t.flags = flags
+	t.options = options
 	return t.data, nil
 }
 
diff --git a/errors.go b/errors.go
new file mode 100755
index 000000000..31d27af0d
--- /dev/null
+++ b/errors.go
@@ -0,0 +1,51 @@
+// Copyright (C) 2014 The Protocol Authors.
+
+package protocol
+
+import (
+	"errors"
+)
+
+const (
+	ecNoError int32 = iota
+	ecGeneric
+	ecNoSuchFile
+	ecInvalid
+)
+
+var (
+	ErrNoError    error = nil
+	ErrGeneric          = errors.New("generic error")
+	ErrNoSuchFile       = errors.New("no such file")
+	ErrInvalid          = errors.New("file is invalid")
+)
+
+var lookupError = map[int32]error{
+	ecNoError:    ErrNoError,
+	ecGeneric:    ErrGeneric,
+	ecNoSuchFile: ErrNoSuchFile,
+	ecInvalid:    ErrInvalid,
+}
+
+var lookupCode = map[error]int32{
+	ErrNoError:    ecNoError,
+	ErrGeneric:    ecGeneric,
+	ErrNoSuchFile: ecNoSuchFile,
+	ErrInvalid:    ecInvalid,
+}
+
+func codeToError(errcode int32) error {
+	err, ok := lookupError[errcode]
+	if !ok {
+		return ErrGeneric
+	}
+	return err
+}
+
+func errorToCode(err error) int32 {
+	code, ok := lookupCode[err]
+	if !ok {
+		return ecGeneric
+	}
+	return code
+}
diff --git a/message.go b/message.go
index 91c331903..c2d898944 100644
--- a/message.go
+++ b/message.go
@@ -1,5 +1,6 @@
 // Copyright (C) 2014 The Protocol Authors.
 
+//go:generate -command genxdr go run ../syncthing/Godeps/_workspace/src/github.com/calmh/xdr/cmd/genxdr/main.go
 //go:generate genxdr -o message_xdr.go message.go
 
 package protocol
@@ -78,8 +79,8 @@ type RequestMessage struct {
 }
 
 type ResponseMessage struct {
-	Data  []byte
-	Error int32
+	Data []byte
+	Code int32
 }
 
 type ClusterConfigMessage struct {
diff --git a/message_xdr.go b/message_xdr.go
index 95d72eb1a..c179de769 100644
--- a/message_xdr.go
+++ b/message_xdr.go
@@ -465,13 +465,13 @@ ResponseMessage Structure:
 \                    Data (variable length)                     \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-|                             Error                             |
+|                             Code                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
 
 struct ResponseMessage {
 	opaque Data<>;
-	int Error;
+	int Code;
 }
 
 */
@@ -502,7 +502,7 @@ func (o ResponseMessage) AppendXDR(bs []byte) ([]byte, error) {
 
 func (o ResponseMessage) EncodeXDRInto(xw *xdr.Writer) (int, error) {
 	xw.WriteBytes(o.Data)
-	xw.WriteUint32(uint32(o.Error))
+	xw.WriteUint32(uint32(o.Code))
 	return xw.Tot(), xw.Error()
 }
 
@@ -519,7 +519,7 @@ func (o *ResponseMessage) UnmarshalXDR(bs []byte) error {
 
 func (o *ResponseMessage) DecodeXDRFrom(xr *xdr.Reader) error {
 	o.Data = xr.ReadBytes()
-	o.Error = int32(xr.ReadUint32())
+	o.Code = int32(xr.ReadUint32())
 	return xr.Error()
 }
 
diff --git a/nativemodel_darwin.go b/nativemodel_darwin.go
index 5b4b9be67..502a71f23 100644
--- a/nativemodel_darwin.go
+++ b/nativemodel_darwin.go
@@ -12,23 +12,23 @@ type nativeModel struct {
 	next Model
 }
 
-func (m nativeModel) Index(deviceID DeviceID, folder string, files []FileInfo) {
+func (m nativeModel) Index(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option) {
 	for i := range files {
 		files[i].Name = norm.NFD.String(files[i].Name)
 	}
-	m.next.Index(deviceID, folder, files)
+	m.next.Index(deviceID, folder, files, flags, options)
 }
 
-func (m nativeModel) IndexUpdate(deviceID DeviceID, folder string, files []FileInfo) {
+func (m nativeModel) IndexUpdate(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option) {
 	for i := range files {
 		files[i].Name = norm.NFD.String(files[i].Name)
 	}
-	m.next.IndexUpdate(deviceID, folder, files)
+	m.next.IndexUpdate(deviceID, folder, files, flags, options)
 }
 
-func (m nativeModel) Request(deviceID DeviceID, folder string, name string, offset int64, size int) ([]byte, error) {
+func (m nativeModel) Request(deviceID DeviceID, folder string, name string, offset int64, size int, hash []byte, flags uint32, options []Option) ([]byte, error) {
 	name = norm.NFD.String(name)
-	return m.next.Request(deviceID, folder, name, offset, size)
+	return m.next.Request(deviceID, folder, name, offset, size, hash, flags, options)
 }
 
 func (m nativeModel) ClusterConfig(deviceID DeviceID, config ClusterConfigMessage) {
diff --git a/nativemodel_unix.go b/nativemodel_unix.go
index 2fb1654c7..21585e308 100644
--- a/nativemodel_unix.go
+++ b/nativemodel_unix.go
@@ -10,16 +10,16 @@ type nativeModel struct {
 	next Model
 }
 
-func (m nativeModel) Index(deviceID DeviceID, folder string, files []FileInfo) {
-	m.next.Index(deviceID, folder, files)
+func (m nativeModel) Index(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option) {
+	m.next.Index(deviceID, folder, files, flags, options)
 }
 
-func (m nativeModel) IndexUpdate(deviceID DeviceID, folder string, files []FileInfo) {
-	m.next.IndexUpdate(deviceID, folder, files)
+func (m nativeModel) IndexUpdate(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option) {
+	m.next.IndexUpdate(deviceID, folder, files, flags, options)
 }
 
-func (m nativeModel) Request(deviceID DeviceID, folder string, name string, offset int64, size int) ([]byte, error) {
-	return m.next.Request(deviceID, folder, name, offset, size)
+func (m nativeModel) Request(deviceID DeviceID, folder string, name string, offset int64, size int, hash []byte, flags uint32, options []Option) ([]byte, error) {
+	return m.next.Request(deviceID, folder, name, offset, size, hash, flags, options)
 }
 
 func (m nativeModel) ClusterConfig(deviceID DeviceID, config ClusterConfigMessage) {
diff --git a/nativemodel_windows.go b/nativemodel_windows.go
index d4feea326..072e1278b 100644
--- a/nativemodel_windows.go
+++ b/nativemodel_windows.go
@@ -24,23 +24,30 @@ type nativeModel struct {
 	next Model
 }
 
-func (m nativeModel) Index(deviceID DeviceID, folder string, files []FileInfo) {
-	for i, f := range files {
-		if strings.ContainsAny(f.Name, disallowedCharacters) {
-			if f.IsDeleted() {
-				// Don't complain if the file is marked as deleted, since it
-				// can't possibly exist here anyway.
-				continue
-			}
-			files[i].Flags |= FlagInvalid
-			l.Warnf("File name %q contains invalid characters; marked as invalid.", f.Name)
-		}
-		files[i].Name = filepath.FromSlash(f.Name)
-	}
-	m.next.Index(deviceID, folder, files)
+func (m nativeModel) Index(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option) {
+	fixupFiles(files)
+	m.next.Index(deviceID, folder, files, flags, options)
 }
 
-func (m nativeModel) IndexUpdate(deviceID DeviceID, folder string, files []FileInfo) {
+func (m nativeModel) IndexUpdate(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option) {
+	fixupFiles(files)
+	m.next.IndexUpdate(deviceID, folder, files, flags, options)
+}
+
+func (m nativeModel) Request(deviceID DeviceID, folder string, name string, offset int64, size int, hash []byte, flags uint32, options []Option) ([]byte, error) {
+	name = filepath.FromSlash(name)
+	return m.next.Request(deviceID, folder, name, offset, size, hash, flags, options)
+}
+
+func (m nativeModel) ClusterConfig(deviceID DeviceID, config ClusterConfigMessage) {
+	m.next.ClusterConfig(deviceID, config)
+}
+
+func (m nativeModel) Close(deviceID DeviceID, err error) {
+	m.next.Close(deviceID, err)
+}
+
+func fixupFiles(files []FileInfo) {
 	for i, f := range files {
 		if strings.ContainsAny(f.Name, disallowedCharacters) {
 			if f.IsDeleted() {
@@ -53,18 +60,4 @@ func (m nativeModel) IndexUpdate(deviceID DeviceID, folder string, files []FileI
 		}
 		files[i].Name = filepath.FromSlash(files[i].Name)
 	}
-	m.next.IndexUpdate(deviceID, folder, files)
-}
-
-func (m nativeModel) Request(deviceID DeviceID, folder string, name string, offset int64, size int) ([]byte, error) {
-	name = filepath.FromSlash(name)
-	return m.next.Request(deviceID, folder, name, offset, size)
-}
-
-func (m nativeModel) ClusterConfig(deviceID DeviceID, config ClusterConfigMessage) {
-	m.next.ClusterConfig(deviceID, config)
-}
-
-func (m nativeModel) Close(deviceID DeviceID, err error) {
-	m.next.Close(deviceID, err)
 }
diff --git a/protocol.go b/protocol.go
index e7b6fe275..b9859203a 100644
--- a/protocol.go
+++ b/protocol.go
@@ -35,6 +35,7 @@ const (
 	stateIdxRcvd
 )
 
+// FileInfo flags
 const (
 	FlagDeleted              uint32 = 1 << 12
 	FlagInvalid                     = 1 << 13
@@ -48,6 +49,17 @@ const (
 	SymlinkTypeMask = FlagDirectory | FlagSymlinkMissingTarget
 )
 
+// IndexMessage message flags (for IndexUpdate)
+const (
+	FlagIndexTemporary uint32 = 1 << iota
+)
+
+// Request message flags
+const (
+	FlagRequestTemporary uint32 = 1 << iota
+)
+
+// ClusterConfigMessage.Folders.Devices flags
 const (
 	FlagShareTrusted  uint32 = 1 << 0
 	FlagShareReadOnly        = 1 << 1
@@ -66,11 +78,11 @@ type pongMessage struct{ EmptyMessage }
 
 type Model interface {
 	// An index was received from the peer device
-	Index(deviceID DeviceID, folder string, files []FileInfo)
+	Index(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option)
 	// An index update was received from the peer device
-	IndexUpdate(deviceID DeviceID, folder string, files []FileInfo)
+	IndexUpdate(deviceID DeviceID, folder string, files []FileInfo, flags uint32, options []Option)
 	// A request was made by the peer device
-	Request(deviceID DeviceID, folder string, name string, offset int64, size int) ([]byte, error)
+	Request(deviceID DeviceID, folder string, name string, offset int64, size int, hash []byte, flags uint32, options []Option) ([]byte, error)
 	// A cluster configuration message was received
 	ClusterConfig(deviceID DeviceID, config ClusterConfigMessage)
 	// The peer device closed the connection
@@ -80,9 +92,9 @@ type Model interface {
 type Connection interface {
 	ID() DeviceID
 	Name() string
-	Index(folder string, files []FileInfo) error
-	IndexUpdate(folder string, files []FileInfo) error
-	Request(folder string, name string, offset int64, size int) ([]byte, error)
+	Index(folder string, files []FileInfo, flags uint32, options []Option) error
+	IndexUpdate(folder string, files []FileInfo, flags uint32, options []Option) error
+	Request(folder string, name string, offset int64, size int, hash []byte, flags uint32, options []Option) ([]byte, error)
 	ClusterConfig(config ClusterConfigMessage)
 	Statistics() Statistics
 }
@@ -169,7 +181,7 @@ func (c *rawConnection) Name() string {
 }
 
 // Index writes the list of file information to the connected peer device
-func (c *rawConnection) Index(folder string, idx []FileInfo) error {
+func (c *rawConnection) Index(folder string, idx []FileInfo, flags uint32, options []Option) error {
 	select {
 	case <-c.closed:
 		return ErrClosed
@@ -177,15 +189,17 @@ func (c *rawConnection) Index(folder string, idx []FileInfo) error {
 	}
 	c.idxMut.Lock()
 	c.send(-1, messageTypeIndex, IndexMessage{
-		Folder: folder,
-		Files:  idx,
+		Folder:  folder,
+		Files:   idx,
+		Flags:   flags,
+		Options: options,
 	})
 	c.idxMut.Unlock()
 	return nil
 }
 
 // IndexUpdate writes the list of file information to the connected peer device as an update
-func (c *rawConnection) IndexUpdate(folder string, idx []FileInfo) error {
+func (c *rawConnection) IndexUpdate(folder string, idx []FileInfo, flags uint32, options []Option) error {
 	select {
 	case <-c.closed:
 		return ErrClosed
@@ -193,15 +207,17 @@ func (c *rawConnection) IndexUpdate(folder string, idx []FileInfo) error {
 	}
 	c.idxMut.Lock()
 	c.send(-1, messageTypeIndexUpdate, IndexMessage{
-		Folder: folder,
-		Files:  idx,
+		Folder:  folder,
+		Files:   idx,
+		Flags:   flags,
+		Options: options,
 	})
 	c.idxMut.Unlock()
 	return nil
 }
 
 // Request returns the bytes for the specified block after fetching them from the connected peer.
-func (c *rawConnection) Request(folder string, name string, offset int64, size int) ([]byte, error) {
+func (c *rawConnection) Request(folder string, name string, offset int64, size int, hash []byte, flags uint32, options []Option) ([]byte, error) {
 	var id int
 	select {
 	case id = <-c.nextID:
@@ -218,10 +234,13 @@ func (c *rawConnection) Request(folder string, name string, offset int64, size i
 	c.awaitingMut.Unlock()
 
 	ok := c.send(id, messageTypeRequest, RequestMessage{
-		Folder: folder,
-		Name:   name,
-		Offset: offset,
-		Size:   int32(size),
+		Folder:  folder,
+		Name:    name,
+		Offset:  offset,
+		Size:    int32(size),
+		Hash:    hash,
+		Flags:   flags,
+		Options: options,
 	})
 	if !ok {
 		return nil, ErrClosed
@@ -280,11 +299,6 @@ func (c *rawConnection) readerLoop() (err error) {
 
 		switch msg := msg.(type) {
 		case IndexMessage:
-			if msg.Flags != 0 {
-				// We don't currently support or expect any flags.
-				return fmt.Errorf("protocol error: unknown flags 0x%x in Index(Update) message", msg.Flags)
-			}
-
 			switch hdr.msgType {
 			case messageTypeIndex:
 				if c.state < stateCCRcvd {
@@ -301,10 +315,6 @@ func (c *rawConnection) readerLoop() (err error) {
 			}
 
 		case RequestMessage:
-			if msg.Flags != 0 {
-				// We don't currently support or expect any flags.
-				return fmt.Errorf("protocol error: unknown flags 0x%x in Request message", msg.Flags)
-			}
 			if c.state < stateIdxRcvd {
 				return fmt.Errorf("protocol error: request message in state %d", c.state)
 			}
@@ -460,16 +470,16 @@ func (c *rawConnection) readMessage() (hdr header, msg encodable, err error) {
 
 func (c *rawConnection) handleIndex(im IndexMessage) {
 	if debug {
-		l.Debugf("Index(%v, %v, %d files)", c.id, im.Folder, len(im.Files))
+		l.Debugf("Index(%v, %v, %d file, flags %x, opts: %s)", c.id, im.Folder, len(im.Files), im.Flags, im.Options)
 	}
-	c.receiver.Index(c.id, im.Folder, filterIndexMessageFiles(im.Files))
+	c.receiver.Index(c.id, im.Folder, filterIndexMessageFiles(im.Files), im.Flags, im.Options)
 }
 
 func (c *rawConnection) handleIndexUpdate(im IndexMessage) {
 	if debug {
-		l.Debugf("queueing IndexUpdate(%v, %v, %d files)", c.id, im.Folder, len(im.Files))
+		l.Debugf("queueing IndexUpdate(%v, %v, %d files, flags %x, opts: %s)", c.id, im.Folder, len(im.Files), im.Flags, im.Options)
 	}
-	c.receiver.IndexUpdate(c.id, im.Folder, filterIndexMessageFiles(im.Files))
+	c.receiver.IndexUpdate(c.id, im.Folder, filterIndexMessageFiles(im.Files), im.Flags, im.Options)
 }
 
 func filterIndexMessageFiles(fs []FileInfo) []FileInfo {
@@ -499,10 +509,11 @@ func filterIndexMessageFiles(fs []FileInfo) []FileInfo {
 }
 
 func (c *rawConnection) handleRequest(msgID int, req RequestMessage) {
-	data, _ := c.receiver.Request(c.id, req.Folder, req.Name, int64(req.Offset), int(req.Size))
+	data, err := c.receiver.Request(c.id, req.Folder, req.Name, int64(req.Offset), int(req.Size), req.Hash, req.Flags, req.Options)
 
 	c.send(msgID, messageTypeResponse, ResponseMessage{
 		Data: data,
+		Code: errorToCode(err),
 	})
 }
 
@@ -510,7 +521,7 @@ func (c *rawConnection) handleResponse(msgID int, resp ResponseMessage) {
 	c.awaitingMut.Lock()
 	if rc := c.awaiting[msgID]; rc != nil {
 		c.awaiting[msgID] = nil
-		rc <- asyncResult{resp.Data, nil}
+		rc <- asyncResult{resp.Data, codeToError(resp.Code)}
 		close(rc)
 	}
 	c.awaitingMut.Unlock()
diff --git a/protocol_test.go b/protocol_test.go
index c1048cdcf..3ff1042cb 100644
--- a/protocol_test.go
+++ b/protocol_test.go
@@ -229,10 +229,10 @@ func TestClose(t *testing.T) {
 		t.Error("Ping should not return true")
 	}
 
-	c0.Index("default", nil)
-	c0.Index("default", nil)
+	c0.Index("default", nil, 0, nil)
+	c0.Index("default", nil, 0, nil)
 
-	if _, err := c0.Request("default", "foo", 0, 0); err == nil {
+	if _, err := c0.Request("default", "foo", 0, 0, nil, 0, nil); err == nil {
 		t.Error("Request should return an error")
 	}
 }
diff --git a/wireformat.go b/wireformat.go
index 4eab3d37e..9411955ba 100644
--- a/wireformat.go
+++ b/wireformat.go
@@ -20,7 +20,7 @@ func (c wireFormatConnection) Name() string {
 	return c.next.Name()
 }
 
-func (c wireFormatConnection) Index(folder string, fs []FileInfo) error {
+func (c wireFormatConnection) Index(folder string, fs []FileInfo, flags uint32, options []Option) error {
 	var myFs = make([]FileInfo, len(fs))
 	copy(myFs, fs)
 
@@ -28,10 +28,10 @@ func (c wireFormatConnection) Index(folder string, fs []FileInfo) error {
 		myFs[i].Name = norm.NFC.String(filepath.ToSlash(myFs[i].Name))
 	}
 
-	return c.next.Index(folder, myFs)
+	return c.next.Index(folder, myFs, flags, options)
 }
 
-func (c wireFormatConnection) IndexUpdate(folder string, fs []FileInfo) error {
+func (c wireFormatConnection) IndexUpdate(folder string, fs []FileInfo, flags uint32, options []Option) error {
 	var myFs = make([]FileInfo, len(fs))
 	copy(myFs, fs)
 
@@ -39,12 +39,12 @@ func (c wireFormatConnection) IndexUpdate(folder string, fs []FileInfo) error {
 		myFs[i].Name = norm.NFC.String(filepath.ToSlash(myFs[i].Name))
 	}
 
-	return c.next.IndexUpdate(folder, myFs)
+	return c.next.IndexUpdate(folder, myFs, flags, options)
 }
 
-func (c wireFormatConnection) Request(folder, name string, offset int64, size int) ([]byte, error) {
+func (c wireFormatConnection) Request(folder, name string, offset int64, size int, hash []byte, flags uint32, options []Option) ([]byte, error) {
 	name = norm.NFC.String(filepath.ToSlash(name))
-	return c.next.Request(folder, name, offset, size)
+	return c.next.Request(folder, name, offset, size, hash, flags, options)
 }
 
 func (c wireFormatConnection) ClusterConfig(config ClusterConfigMessage) {