diff --git a/sharing/proto/wire_format.proto b/sharing/proto/wire_format.proto
index 5a166991..2d35c6a7 100644
--- a/sharing/proto/wire_format.proto
+++ b/sharing/proto/wire_format.proto
@@ -183,7 +183,7 @@ message Frame {
   optional V1Frame v1 = 2;
 }
 
-// NEXT_ID=8
+// NEXT_ID=9
 message V1Frame {
   enum FrameType {
     UNKNOWN_FRAME_TYPE = 0;
@@ -196,6 +196,7 @@ message V1Frame {
     CANCEL = 6;
     // No longer used.
     PROGRESS_UPDATE = 7;
+    FILE_SYNC = 8;
   }
 
   optional FrameType type = 1;
@@ -207,6 +208,7 @@ message V1Frame {
   optional PairedKeyResultFrame paired_key_result = 5;
   optional CertificateInfoFrame certificate_info = 6 [deprecated = true];
   optional ProgressUpdateFrame progress_update = 7 [deprecated = true];
+  optional SyncFrame file_sync = 8;
 }
 
 // An introduction packet sent by the sending side. Contains a list of files
@@ -241,6 +243,43 @@ message ProgressUpdateFrame {
   optional bool start_transfer = 2;
 }
 
+// A packet for file sync messages.
+// NEXT_ID=3
+message SyncFrame {
+  oneof content {
+    SyncHandshake handshake = 1;
+    SyncConfig config = 2;
+  }
+}
+
+// A packet for file sync handshake messages.
+// NEXT_ID=1
+message SyncHandshake {}
+
+// A packet for file sync config messages.
+// NEXT_ID=2
+message SyncConfig {
+  repeated SyncFolder folders = 1;
+}
+
+// A packet for file sync folder messages.
+// NEXT_ID=5
+message SyncFolder {
+  // An identifier of the folder for the pair of source and target devices to
+  // uniquely identify it among all folders that are being synced.
+  optional string id = 1;
+  // Human readable name of the folder.
+  optional string label = 2;
+  // A randomly generated id when the index is created. Regenerate when the
+  // index is reset.
+  optional int32 index_id = 3;
+  // The maximum sequence number of the folder. Each number represents an update
+  // to a file in the folder. Sequence numbers are only valid within the scope
+  // of a valid index_id. If an index is reset, all sequence numbers need to be
+  // regenerated, including max_sequence.
+  optional int64 max_sequence = 4;
+}
+
 // A response packet sent by the receiving side. Accepts or rejects the list of
 // files.
 // NEXT_ID=4