Page 194 - DCAP406_DCAP_207_Computer Networks

Unit 12: Application Layer

2. Mail: It is a standard (STD 11) defining the format of the mail messages, syntax of mail Notes
header fields, a set of header fields and their interpretation and about a set of document
types other than plain text ASCII to be used in the mail body.
3. DNS-MX: It is a standard for the routing of mail using the Domain Name System
(RFC 974).

SMTP, an application layer protocol, is used to send e-mail messages across the Internet.
It utilizes TCP as the transport protocol to send e-mail to a destination mail exchanger, referred
as mail server. A client machine sends e-mail to a mail exchanger or an e-mail is sent from mail
exchanger to another mail exchanger. E-mail transmitted using SMTP is normally transmitted
from one mail exchanger to another directly. E-mail was never designed to be instantaneous but
it appears so often.

Notes Mail Exchangers are nothing but the software application programs to support the
SMTP protocol. Mail Exchangers such as sendmail or Microsoft Exchange wait for IP
datagrams that arrive on the network interface with a TCP port number of 25. When a
message is arrived, the mail exchanger checks to find out if it is for one of its users and
accordingly move the mail to the user’s mailbox. The data sent using SMTP is 7-bit ASCII
data, with the high-order bit cleared to zero is found adequate in most instances for the
transmission of English text messages but is inadequate for non-English text or non-
textual data. To overcome these limitations, Multipurpose Internet Mail Extensions (MIME)
defines a mechanism for encoding text and binary data as 7-bit ASCII within the mail
envelope and SMTP Service Extensions specifies a mechanism to extend the capabilities of
SMTP beyond the limitations.

How SMTP Works?

SMTP is end-to-end delivery in which an SMTP client machine contacts the destination host’s
SMTP server directly to deliver the mail. Unlike the store-and-forward principle that delivers
the mail content to the destination host through a number of intermediary nodes in the same
network, SMTP continues the mail content being transmitted until it has been successfully
copied to the host’s SMTP. In case of store and forward mechanism, the successful transmission
from the sender only indicates that the mail content has reached the first intermediate hop.
There are instances when mail is exchanged between the TCP/IP SMTP mailing system and the
locally used mailing systems. Such applications are referred as mail gateways or mail bridges.
However, SMTP guarantees only delivery to the mail-gateway host, not to the real destination
host, which is located beyond the TCP/IP network. In case of a mail gateway, the SMTP end-to-
end transmission is host-to-gateway, gateway-to-host or gateway-to-gateway. SMTP does not
specify the format of mail beyond the gateway.
Each message of SMTP contains the following fields:
A header or envelope that is terminated by a null line.

Contents - Everything after the null or blank line is the message body with sequence of
lines containing ASCII characters.
Simple Mail Transfer Protocol defines a client/server protocol. The client SMTP machine initiates
the session by sending SMTP message and the mail server responds by receiving SMTP message
to the session request.

LOVELY PROFESSIONAL UNIVERSITY 187