Protocol Sniffing: A Case Study

At one point in time, all user access to computing facilities in the organization under study
(the university at which the author is employed) was done via terminals. It was not practical to
hardwire each terminal to the host, and users needed to use more than one host. To solve these
two problems, Central Computing used a switch (an AT&T ISN switch) between the terminals
and the hosts. The terminals connected to the switch so that the user had a choice of
hosts. When the user chose a host the switch connected the terminal to the chosen host via a
very real, physical connection. The switch had several thousand ports and was, in theory,
capable of setting up connections between any pair of ports. In practice, however, some ports
attached to terminals and other ports attached to hosts. Figure 6.1 illustrates this setup.
Figure 6.1
Case study system
before networking.
~2500 Input
~400 Output
[SN Switcher]
IBM Mainframe
DEC Vax
DEC Vax
Multiplexor
To make the system more flexible, the central computing facility was changed to a new system
that uses a set of (DEC 550) Ethernet terminal servers with ports connected to the switch,
rather than the old system, which used a fixed number of switch ports connected to each host.
The new terminal servers are on an Ethernet segment shared by the hosts in the central
machine room.