Information Technology Reference
In-Depth Information
has been extended to include support for file attachments beyond simple
ASCII text, sender/recipient authentication, and transport-layer security.
Very early forms of e-mail evolved as a means for systems administra-
tors to leave one-another messages, and the SMTP protocol's evolution
reflects this origin.
The first e-mail systems operated exclusively within a single mainframe
computer, although the development of network connections between
mainframe systems was rapidly met by a network-capable version of the
messaging system. Because early networking was not always-on, transfer
of messages from one node to the next required a messaging protocol able
to queue a message for transfer the next time that network connectivity
was available. This type of transport is known as “store and forward”
transfer. It was also necessary for messages that waited too long for trans-
fer to be reported back to the originator as undeliverable.
Early forms of e-mail did not enjoy the automatic routing provided by
the Internet's hierarchical Domain Name System (DNS) and were instead
directed from one node to the next by the message sender and system admin-
istrators along the transmission path. An early e-mail address might have
been:
bob.admin!bobscomputer!joescomputer!bettyscomputer
, so that the mes-
sage would be sent through each server in turn until it reached its intended
recipient. If a node in the specified path remained unavailable, this form of
routing could not adapt and redirect through an alternate path.
The SMTP protocol is more robust than the early forms of e-mail
routing, allowing an e-mail message to travel along any available route.
If a node fails along the path, the previous SMTP server can simply retry
along an alternate route, as shown in Figure 6.5.
E-mail in Critical Communications
Electronic mail transfer is very durable and is able to overcome tempo-
rary network changes. This same durability, however, affects its use in
time-sensitive or content-sensitive communications. Emergency notifica-
tion systems in particular cannot rely solely on SMTP transport, because
durability of transmission does not equate to urgency of delivery. Time-
liness suffers in SMTP-based notification systems, requiring additional
mechanisms for urgent alert notification.
E-mail is very valuable in providing continuity of operations dur-
ing disaster recovery efforts or during other times when the workforce is
