diff --git a/doc/api.org b/doc/api.org
index daa6e5b..cd7dec3 100644
--- a/doc/api.org
+++ b/doc/api.org
@@ -19,18 +19,22 @@
       #+BEGIN_SRC json
         {
             "type": "state",
+            "syn": 1,
             "game-id": 1,
             "game-over": false,
             "snake": [(1,2),(1,3)],
             "food": [(6,7)]
         }  
       #+END_SRC
+      Note that, syn entry is used to keep packet ordering consistent and detecting packet inversion on the network. Thus,
+      syn entry indicate the expected syn that the client should send on the next UDP packet.
 ** Gameplay
 *** Change Direction
     1. When client is playing a game it can ask to the server to change snake direction:
        #+BEGIN_SRC json
          {
              "type": "update",
+             "syn": 1,
              "game-id": 1,
              "direction": "left",
          }
@@ -39,6 +43,7 @@
        #+BEGIN_SRC json
          {
              "type": "state",
+             "syn": 2,
              "game-id": 1,
              "game-over": false,
              "snake": [(0,2),(1,2)],
@@ -50,6 +55,7 @@
        #+BEGIN_SRC json
          {
              "type": "update",
+             "syn": 2,
              "game-id": 1,
              "direction": null
          }
@@ -58,6 +64,7 @@
        #+BEGIN_SRC json
          {
              "type": "state",
+             "syn": 3,
              "game-id": 1,
              "game-over": false,
              "snake": [(1,2),(0,2)],
@@ -69,10 +76,11 @@
        #+BEGIN_SRC json
          {
              "type": "state",
+             "syn": null,
              "game-id": 1,
              "game-over": true,
              "snake": [(0,2),(1,2)],
              "food": [(6,7)]
          }  
        #+END_SRC
-    - No reply is expected from the client and server will be in charge to free local memory
+    - No reply is expected from the client and server will be in charge to free local memory. Note that syn=null.
diff --git a/doc/api.pdf b/doc/api.pdf
index a1f94e5..a02a350 100644
Binary files a/doc/api.pdf and b/doc/api.pdf differ