All computers on the Internet are connected, but not all connections are
equal. Hosts are grouped into islands of local communication. It is the agreed
conventions and shared knowledge that connect these islands, just as much as the
switches and wires that run between them.
The power and limitation of these conventions and shared knowledge and
hence their effectiveness can be investigated by an appropriate calculus. In this
thesis I describe a development of the 7r-calculus that is particularly well suited to
express such systems. The process calculus, which I call the local area n-calculus
or Ian, extends the 7r-calculus so that a channel name can have within its scope
several disjoint local areas. Such a channel name may be used for communication
within an area or it may be sent between areas, but it cannot itself be used to
transmit information from one area to another. Areas are arranged in a hierarchy
of levels which distinguish, for example, between a single application, a machine,
or a whole network. I present a semantics for this calculus that relies on several
side-conditions which are essentially runtime level checks. I show that a suitable
type system can provide enough static information to make most of these checks
unnecessary.
I examine the descriptive power of the /a7r-calculus by comparing it to the
7r-calculus. I find that, perhaps surprisingly, local area communication can be
encoded into the 7T-calculus with conditional matching. The encoding works by
replacing communication inside an area with communication on a new channel
created just for that area. This is analogous to replacing direct communication
between two points with a system that broadcasts packets over a background
ether. I show a form of operational correspondence between the behaviour of a
process in lan and its 7r-calculus translation.
One of my aims in developing this calculus is to provide a convenient and ex¬
pressive framework with which to examine convention-laden, distributed systems.
I offer evidence that the calculus has achieved this by way of an extended case
study. I present a model of Internet communication based on Sockets and TCP
over IP and then extend this system with Network Address Translation. I then
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give a model of the File Transfer Protocol that uses TCP/IP to communicate
between networks.
Traces of the model show that FTP, run in its normal mode, will fail when
the client is using Network Address Translation, whereas, an alternative mode of
FTP will succeed. Moreover a normal run of the model over NAT fails in the
same way as the real life system would, demonstrating that the model can pick
up this failure and correctly highlight the reasons behind it
