Computer Mail

This allows you to send messages to users on other
computers. Originally, people tended to use only one or two specific
computers. They would maintain "mail files" on those machines. The
computer mail system is simply a way for you to add a message to
another user's mail file. There are some problems with this in an
environment where microcomputers are used. The most serious is that a
micro is not well suited to receive computer mail. When you send mail,
the mail software expects to be able to open a connection to the
addressee's computer, in order to send the mail. If this is a
microcomputer, it may be turned off, or it may be running an
application other than the mail system. For this reason, mail is
normally handled by a larger system, where it is practical to have a
mail server running all the time. Microcomputer mail software then
becomes a user interface that retrieves mail from the mail
server. (See RFC 821 and 822 for specifications for computer mail. See
RFC 937 for a protocol designed for microcomputers to use in reading
mail from a mail server.)
These services should be present in any implementation of TCP/IP, except that micro-
oriented implementations may not support computer mail. These traditional applications
still play a very important role in TCP/IP-based networks. However more recently, the
way in which networks are used has been changing. The older model of a number of
large, self-sufficient computers is beginning to change. Now many installations have
several kinds of computers, including microcomputers, workstations, minicomputers, and
mainframes. These computers are likely to be configured to perform specialized
tasks. Although people are still likely to work with one specific computer, that computer
will call on other systems on the net for specialized services. This has led to the
"server/client" model of network services. A server is a system that provides a specific
service for the rest of the network. A client is another system that uses that service. (Note
that the server and client need not be on different computers. They could be different
programs running on the same computer.)
Here are the kinds of servers typically present in a modern computer setup. Note that
these computer services can all be provided within the framework of TCP/IP.