2011 Paper 5 Question 5

2011 Paper 5 Question 5
2011 Paper 5 Question 5
Computer Networking
Consider two physically-separated entities A and B. B has been supplied messages
that will be sent to A following these conventions:
• A gets a request from the layer above to retrieve the next data (D) message
from B.
• A must send a request (R) message to B on the A-to-B channel.
• Upon receipt of an R, B will send D back to A on the B-to-A channel.
• A should deliver exactly one copy of each D message to the layer above.
• R messages may be lost (but will not be corrupted) in the A-to-B channel.
• D messages are always delivered correctly (no loss or corruption).
• The delay along each channel is unknown and variable.
Give the FSM describing a protocol employed by A and B.
This FSM must compensate for the loss-prone channel between A and B, as well
as implementing message passing to the layer above at entity A. Your FSM must
not use more mechanisms than is necessary.
[20 marks]
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