6)Answer:
timed asynchronous system model.:
The timed asynchronous (TA) model is an accurate description of the asynchronous distributed systems with a communication network which has a non-negligible failure frequency.
The timed asynchronous (TA) system model [Cri99] can be used to describe existing distributed systems built from network workstations connected by unreliable communication networks. It allows many needed services such as clock synchronization, membership, consensus etc. to be implemented
More and more distributed systems are using unreliable communication networks, e.g. systems based on mobile communication are asynchronous by their nature. It is possible to model such systems with an integration of the TA model and TTPs to maintain the wide system coverage of the TA model and the cost-effective properties of TTPs at the same time.
Distributed systems can be classified as synchronous or asynchronous depending on whether their underlying communication and process management services can provide “certain communication” or not [Cri96]. A system is synchronous if it guarantees certain communication and is asynchronous otherwise[Cri99]. A certain communication has the property that at any time there is a minimum number of correct processes, and any message sent by a correct process to a correct destination process is received and processed at the destination within a known amount of time. This means the probability the message is not received and processed in time is “negligible”. This property can be achieved upon the assumption that the frequency of failures that can occur in a system is bounded [Cri91]. Unfortunately, in many distributed systems this assumption can not be fully satisfied. The TA model is proposed by Flaviu Cristian and Christof Fetzer to describe asynchronous distributed systems. It makes following basic assumptions on the underlying system [Cri99]: 1) All services are timed: their specification is prescribed not only in the value domain, but also in the temporal domain; 2) Interprocess communication is via an unreliable datagram service with omission/performance failure semantics: the only failures that messages can suffer are omission (message is dropped) and performance failures (message is delivered late); 3) Processes have crash/performance failure semantics: the only failures a process can suffer are crash and performance failures; 4) Processes have access to hardware clocks that run within a linear envelope of real-time; and 5) No bound exists on the rate of communication and process failures that can occur in a system. These assumptions are practical because: 1) Nearly all the workstations now are using high-precision quartz clocks; and 2) For those services without any response time promises, a high level abstraction that depends on them (it could be a human user at the highest level) can define a timeout to decide if they are failed. This model adequately describes existing distributed systems build from network workstations since many practically needed services such as clock synchronization, membership, consensus, election, and atomic broadcast have been shown to be implementable [Cri99] [Fet99]. In [Fet96] the notion of fail-awareness is introduced as a systematic means of transforming synchronous service specifications into fail-aware specifications that become implementable in timed asynchronous systems.
A timed asynchronous distributed system consists of a finite set of processes which communicate via a datagram service. Processes run on the computer nodes of a network. Lower level software in the nodes and the network implements the datagram service. Each process has access to a local hardware clock. The process management service that runs in each node uses this clock to manage alarm clocks that allow the local processes to request to be awakened whenever desired [Cri99]. The hardware clock consists of an oscillator and a counting register that is increased by the ticks of the oscillator. It can drift apart from real-time because of the imprecision of the oscillator, temperature changes, and aging. The model assumes there exists a constant maximum drift rate that bounds the absolute value of the drift rate of a correct clock. Hence in this model, all the correct clocks are within a narrow linear envelope of realtime. The drift rate can be further divided into systematic drift error due to the imprecision of the oscillator and drift errors due to other reasons such as aging or changes in the environment. A hardware clock can be calibrated by multiplying a constant factor to reduce the systematic drift error and it could reduce the drift rate by a magnitude of two in prac-tise. Clocks can be externally synchronized if at any instant the deviation between any correct clock and real-time is bounded by a known constant, or internally synchronized if the deviation between any two correct clocks is bound by a known constant. Either of these synchronization needs to be performed periodically to account for the ongoing drift of all clocks [Cri99]. The datagram service provides primitives for transmitting unicast and broadcast messages. It must satisfy following requirements: 1) Validity: if the datagram service delivers a message m to a process p at time t and identifies process q as m’s sender, then q must sent m at some earlier time s
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