Any client using the ntp service must communicate with it by means of messages passed over a communication channel. If a bound can be set on the time to transmit a message over a communication channel, then the difference between the client’s clock and the value supplied by the ntp service would also be bounded. With unbounded message transmission time, clock differences are necessarily unbounded. Consider two communication services for use in asynchronous distributed systems. In service A, messages may be lost, duplicated or delayed and checksums apply only to headers. In service B, messages may be lost. delayed or delivered too fast for the recipient to handle them, but those that are delivered arrive order and with the correct contents.
Describe the classes of failure exhibited by each service. Classify their failures according to their effect on the properties of validity and integrity. Can service B be described as a reliable
communication service?
Service A can have:
arbitrary failures:
– as checksums do not apply to message bodies, message bodies can corrupted.
– duplicated messages,
omission failures (lost messages).
Because the distributed system in which it is used is asynchronous, it cannot suffer from timing failures.
Validity - is denied by lost messages
Integrity - is denied by corrupted messages and duplicated messages. Service B can have:
omission failures (lost messages, dropped messages).
Because the distributed system in which it is used is asynchronous, it cannot suffer from timing failures. It passes the integrity test, but not the validity test, therefore it cannot be called reliable.
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