diff --git a/protocol/PROTOCOL.md b/protocol/PROTOCOL.md
index 420d332d8..d0cc412af 100644
--- a/protocol/PROTOCOL.md
+++ b/protocol/PROTOCOL.md
@@ -404,6 +404,11 @@ The Flags field is made up of the following single bit flags:
    synchronization. A peer MAY set this bit to indicate that it can
    temporarily not serve data for the file.
 
+ - Bit 17 ("P") is set when there is no permission information for the
+   file. This is the case when it originates on a non-permission-
+   supporting file system. Changes to only permission bits should be
+   disregarded on files with this bit set.
+
  - Bit 0 through 17 are reserved for future use and SHALL be set to
    zero.
 
diff --git a/protocol/protocol.go b/protocol/protocol.go
index 767329c6a..55bc47352 100644
--- a/protocol/protocol.go
+++ b/protocol/protocol.go
@@ -25,9 +25,10 @@ const (
 )
 
 const (
-	FlagDeleted   uint32 = 1 << 12
-	FlagInvalid          = 1 << 13
-	FlagDirectory        = 1 << 14
+	FlagDeleted    uint32 = 1 << 12
+	FlagInvalid           = 1 << 13
+	FlagDirectory         = 1 << 14
+	FlagNoPermBits        = 1 << 15
 )
 
 const (