diff --git a/DOCS/tech/mpcf.txt b/DOCS/tech/mpcf.txt
index 662634f898..b31668c7f8 100644
--- a/DOCS/tech/mpcf.txt
+++ b/DOCS/tech/mpcf.txt
@@ -310,6 +310,8 @@ note, demuxers MUST be able to deal with new and unknown headers
 packet_header
         startcode                               f(64)
         forward_ptr                             v
+        if(forward_ptr > 4096)
+            header_checksum                     u(32)
 
 packet_footer
         reserved_bytes
@@ -589,6 +591,9 @@ checksum
     for frame headers the checksum contains the framecode byte and all
     following bytes upto the checksum itself
 
+header_checksum
+    checksum over the startcode and forward pointer
+
 Syncpoint tags:
 ---------------