103
Chapter 8:
Understanding Networking Protocols
and typically runs on port 23 (although it can run on any port that has been enabled
on the server system). Once users connect using Telnet, they must log in to the remote
system using the same credentials they would use if they were working from a directly
connected terminal.
Simple Mail Transfer Protocol (SMTP)
E-mail had a somewhat rocky start on the Internet, with early e-mail programs sharing
few standards with other e-mail programs, particularly in the handling of attached
binary data. The good news is that the situation is now resolved, and all current e-mail
software supports all the widely accepted standards.
The Simple Mail Transfer Protocol (SMTP) is used to send and receive e-mail
messages from one e-mail server to another. The SMTP protocol defines a dialog
between a sending system and a receiving system.
An SMTP dialog starts when a sending system connects to port 25 of a receiving
system. After the connection is established, the sending system sends a HELO command,
followed by its address. The receiving system acknowledges the HELO command
along with its own address. The dialog then continues, with the sending system issuing
a command indicating that the system wants to send a message and identifying the
recipient for whom the message is intended. If the receiving system knows of the
recipient, it acknowledges the request, and then the sending system transmits the body
of the message along with any attachments. Finally, the connection between the two
systems is terminated once the receiving system acknowledges that it has received the
entire message. Figure 8-2 illustrates this process.
Figure 8-2.
Part of an SMTP dialog between systems
SMTP
sender
SMTP
receiver
Mail from address
250 OK
250 OK
DATA
354 Start
Data for message
250 OK
Quit
221 Terminating
HELO
250 OK
RCPT TO: e-mail address
Summary :
103 Chapter 8: Understanding Networking Protocols and typically runs on port 23 (although it can run on any port that has been enabled on the server system). An SMTP dialog starts when a sending system connects to port 25 of a receiving system. The dialog then continues, with the sending system issuing a command indicating that the system wants to send a message and identifying the recipient for whom the message is intended.
Tags :
email,smtp,receiing,message,sending,address,dialog,250,acknowledges,between,command,protocol,helo