Designing an Network Protocol

| March 11, 2014

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Aim of the Coursework
This network protocol design coursework is intended to develop your understanding of how a
complex application layer service can be designed using the finite state machine methodology,
how multi-party sessioning techniques are required where multiple distributed processes
must collaborate, and how network failures such as crashing hosts and lost messages must be
considered when creating resilient protocols.
A manufacturer of road maintenance equipment wishes to develop a control protocol to enable
multiple portable traffic light units (TLU) to automatically manage a road junction with
an arbitrary number of roads. Using the protocol specification given overleaf, develop a suitable
finite state machine for controlling each TLU which is capable of interacting with an arbitrary
number of other TLUs over a local wireless subnet.
Your answer should contain (i) a clear description of the protocol including its message types
and and operation including any sequence diagrams, (ii) a suitably annotated finite state machine
for a traffic light unit and its corresponding state transition diagram, and optionally (iii)
a pseudo-code description or a fully implemented programme.
Additional Points
– Note that there is not necessarily a single correct answer.
– If you feel any of the specification below is ambiguous or unclear, feel free to make any sensible
assumptions – though remember to write down these assumptions in your report.
– There are no specific length or word count requirements. However please note the marking
criteria listed below.
The marking scheme will be based on the following criteria:
– 40% A clear description of the protocol including its message types and its operation
including any sequence diagrams.
– 30% A suitably annotated finite state machine diagram for controlling a traffic light
unit and its corresponding state transition diagram.
– 30% Implementation of the protocol in either pseudo-code or a fully implementable
programming language. (A Java and C++ socket and messaging API will be provided
if required.)
Mobile traffic light units (TLU) are free-standing units with their own power and are fitted
with computer, wireless network interface and light controller. (See Figure 1). When road
maintenance work is being conducted at a junction, one of these lights is placed at each approach
road. (See Figure 2.) A TLU controls the traffic flowing into the junction on its approach
Each TLU on the same junction is manually switched on by the installer. As part of this
switch-on process, the installer uses the TLU’s control panel (see Figure 3) to
– select the wireless channel number for the junction’s subnet. All TLUs on the same
junction should be switched to the same channel number.
– specify the number of TLUs operating on this junction, and
– choose a suitable time interval (the pass duration) which traffic travelling in this direction
should be allowed.
After setting the panel to the appropriate values, the installer switches on each TLU on after
the other in no particular order. Once switched on, the TLU remains with its lights off until
instructed by the protocol to do otherwise.
locally by an integer variable TULid. Each TLU can be instructed to
– execute the GO sequence MyLights.Go()in which the lights go through the usual
starting sequence and allow traffic to flow for the TLU’s PassDuration in seconds,
– execute a PASS sequence MyLights.Pass(int Interval) in which the lights go
through the usual starting sequence and allow traffic to flow for the TLU’s PassDuration
in seconds before returning the lights to the stop configuration, or
– execute the STOP sequence MyLights.Stop(), or
– switch off its lights completely MyLights.Off().
Having switched on, the TLUs collaborate to automatically select a MASTER TLU amongst
themselves which will be responsible for
– maintaining a list of TLU IDs operating on the junction,
– instructing each TLU in turn to allow traffic to flow, and
– managing situations where any TLU is no longer available on the subnet.
Once selected, the MASTER TLU instructs all TLUs to go to the STOP configuration for 30
seconds. It then instructs each TLU in turn (including itself) to allow traffic to pass.
All TLUs should be expected to crash and recover periodically but in each case, recovery is
expected to take place within a minute. During this minute, all TLUs should set their lights to
the STOP configuration. If the MASTER TLU detects the loss of a TLU for more than a
minute, all TLUs should be instructed to switch off their lights. If the MASTER TLU crashes,
then the TLUs repeat the process of automatically selecting a MASTER. If at anytime the installer
replaces a broken TLU, normal operation should resume automatically as soon as possible.
You can assume that only TLUs on the same road intersection can communicate with each. Please follow the assignment requirements from a file called Protocol design coursework. The other documents are workshop activities, the writer can use them for support.
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Conceptual Analysis 3


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