TCP/IP is a layered protocol for controlling data flow in a
packet switched network. For a more complete discussion
of TCP/IP see, for example, Hunt (1998). Figure 1 lists the
layers. TCP/IP uses packet switching, which means that
each message is broken into pieces (called packets), each
of which contains part of the message. Each packet is
augmented with a header that contains (among other
things) the addresses of the source and destination ma-
chines. At each step along the way, a router looks at the
header, determines the address of the next host along the
way, and forwards the packet to that next host.
Understanding the addressing presents a major hurdle
to understanding TCP/IP. There are two types of ad-
dresses in each packet. The IP header includes the IP
addresses of the source and destination machines. These
do not change as the packet moves through the network.
DLL layer header includes the MAC addresses of the
source and destination machines for the current hop.
These change at each hop. Since there are two destination
addresses and two source addresses, some students have
difficulty understanding how the addressing works. One
way to help explain the addresses is to have the students
play a game.
As examples of this approach, consider two similar
games, one designed by Dennis (2002) and one by
Pendegraft (2002, 2003), to help teach how TCP/IP works.
Both are published elsewhere and so will not be described
in detail here. Both games are designed to be run in one
class session in a course on telecommunications. Both
seem suitable for undergraduates or graduate students.
Both could be adapted to class sizes ranging from a dozen
to more than 40.
The games have similar structure. The class is divided
into teams, each team representing a host. Each player
represents one layer on that host. A network map and
instructions for each layer are given to the players. Figure
2 shows a sample network, and Figure 3 gives an excerpt
from Dennis’s instructions for the Data Link Layer.
In the play of the simulation, an application layer
player writes a message to another application layer
player on a paper form. The form is then handed to the TCP
player of the sending host. The TCP player adds the TCP
header and hands the packet to the IP player. The IP player
adds the IP header and hands the packet to the DLL player.
The message is then passed to another host where each
player strips off the header for that layer and hands the
message upward, or in the case of IP forwards it as
necessary.
There are several structural differences. Pendegraft
gives the IP layer a routing table (see Figure 4 for an
example), while Dennis has the IP layer use the network
map to determine first hop IP addresses. Pendegraft uses
a separate envelope for each layer and its header, while
Dennis has different forms for each layer’s header data
which are taped to the message form. Another difference
is that in Pendegraft’s game, there is no physical layer
player. Instead, the DLL players hold the ends of pieces
of wire representing the physical layer. When a DLL
player finishes adding the MAC addresses to the packet,
he strings the packet on the wire and by raising his hand,
sends the packet to the next machine. Such silliness
introduces some humor into an otherwise dry subject, and
helps keep the students interested and involved.
The games offer different points of view. Dennis’s
game allows many messages on the network at one time;
consequently, it offers a more complete (and hence more
complex) model of TCP/IP. In Pendegraft’s game only one
message is sent at a time and the entire class follows it
along the way, immediately discussing problems that may
occur such as a player incorrectly addressing a packet. In
Dennis’s game that sort of error is handled in discussion
between the affected players. One result of these two
points of view is that in Dennis’s game, each player has
a different experience that can be shared in the post-game
discussion, while in Pendegraft’s game, there is a more
shared experience.
Both games simplify TCP/IP, ignoring some issues like
handshaking or error detection. This is not to say that
these issues are unimportant, but that these games focus
attention on a limited set of issues of paramount impor-
tance, primarily addressing. Pendegraft’s game is de-
signed to allow including such complexities “notionally.”
That is, the instructor can intervene with an external
complexity. For example, after a couple of messages have
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