Update page 'Protocol'

Protocol.md

@@ -92,6 +92,30 @@ _stream.Write(BitConverter.GetBytes(data.Length), 0, 4); // Send the length
 _stream.Write(data, 0, data.Length); // Send the packet
 ```
 
+## Request and Reply
+
+When a request is sent, the receiver must always respond with the same command name,
+only the arguments change.  
+This allow the sender to know if the receiver responded correctly to its request.
+
+Here is an example with a ping command (without the end `\n`):
+
+```md
+PING
+ID;ping
+```
+
+> Request
+
+```md
+PING
+ID;pong
+```
+
+> Response
+
+Here, a client send a 'ping' request to a server and the server respond with a 'pong', using the same command.
+
 ## Commands
 
 Each command is described in a dedicated page