(Packet) Semantics

Sender and recipient must agree on what the recipient can assume if it receives a particular packet

What actions the recipient should take in response to the packet

May be described using a Finite State Machine (FSM) which represents the transitions involves in the protocol.

Ex. Mail session, using the SMTP Protocol (Simple Mail Transfert Protocol)

localhost.10:29.~ pvk > telnet cs.anu.edu.au 25
Trying 150.203.164.35...
Connected to cs.anu.edu.au.
Escape character is '^]'.
220 cs.anu.edu.au ESMTP Exim 3.35 #1 Mon, 05 Aug 2002 10:28:48 +1000
Helo ruth
250 cs.anu.edu.au Hello box.anu.edu.au [150.203.160.68]
Mail From:<GDAY@MATE.au>
250 <GDAY@MATE.au> is syntactically correct
Rcpt To:<pvk@vuylsteker.net>
250 <pvk@vuylsteker.net> is syntactically correct
Data
354 Enter message, ending with "." on a line by itself
Coucou PVK
How are you ?
.
250 OK id=17bVlN-0003b4-00
QUIT
221 cs.anu.edu.au closing connection
Connection closed by foreign host.

To learn more about SMTP protocol...