Chapter 28

28.1	Read RFC 822 to find out what a domain literal is. Try sending mail to yourself using one.
A:	A domain literal is a dotted -decimal IP address within square brackets. For example: `mail rstevens@ [140.252.1.54].`
28.2	Excluding the connection establishment and termination, what is the minimum number of network round trips to send a small mail message?
A:	Six round trips: the HELO command, MAIL, RCPT, DATA, body of the message, and QUIT.
28.3	TCP is a full-duplex protocol, yet SMTP uses TCP in a half-duplex fashion. The client sends a command then stops and waits for the reply. Why doesn't the client send multiple commands at once, for example, a single write that contains the HELO, MAIL, RCPT, DATA, and QUIT commands ( assuming the body isn't too large)?
A:	This is legal and is called pipelining [Rose 1993, Sec. 4.4.4]. Unfortunately there exist brain-damaged SMTP receiver implementations that clear their input buffer after each command is processed , causing this technique to fail. If this technique is used, naturally the client cannot discard the message until all the replies have been checked to verify that the message was accepted by the server.
28.4	How can this half-duplex operation of SMTP fool the slow start mechanism when the network is running near capacity?
A:	Consider the first five network round trips from Exercise 28.2. Each is a small command (probably a single segment) that places little load on the network. If all five make it through to the server without retransmission, the congestion window could be six segments when the body is sent. If the body is large, the client could send the first six segments at once, which the network might not be able to handle.
28.5	When multiple MX records exist with the same preference value, should they always be returned by a name server in the same order?
A:	Newer releases of BIND shuffle the MX records with the same value, as a form of load balancing.