@item The 128-bit cookie is sent by client over UDP to the server every
second. If UDP packets are lost, then no connection is possible and
-after a timeout the server drops the TCP connection.
-
-@item Otherwise it replies with @code{SID XXX}, where XXX is ASCII
-decimal stream number client must use.
+after a timeout the server drops the TCP connection. That cookie means:
+
+ @itemize
+ @item confirmation of successful handshake on client side;
+ @item UDP hole punching of stateful firewall or NAT;
+ @item fact of client's UDP traffic ability to reach the server;
+ @item client's UDP address knowledge (after passing NAT, its port
+ may differ from known to client one)
+ @end itemize
+
+@item Server replies with @code{SID XXX}, where XXX is ASCII decimal
+stream number client must use.
@item @code{PING} and @code{PONG} messages are then sent every ten
seconds as a heartbeat.