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chapter 10 distributed syncronization, request driven -a request- driven implementation for a centralized physical time service, each participant sends a request to the time service for the current time., 10.1 introduction to global time -a distributed system involves many participating locations and individual computer systems., 10.2 physical clocks -are utilized to convey a sense of people time throughout the distributed system, 10.3.1 ntp architecture -is architecture is organized into a hierarchical tree structure, this structure must be tree and never form a loop., 3. allow frequent resynchronization, 10.2.4 distributed physical time services -the distributed approach for implementing a physical time service requires each location to broadcast its current time at prefined set intervals., 10.4.1 happen-before relationship, 2.enable survival despite significant losses of connectivity., 1. involves obtaining an accurate value for physical time, 10.5 summary -we have focused on issues and solutions to obtaining system-wide synchronization. , 10.2.2 synchronizing physical time -the second aspect of a physical time service involves the synchronization of time throughout the distributed system., 1.the participants evaluate their clock in comparison to the broadcast time received while accommodating the delay due to network travel time., 2.based on implementation of a centralized physical time service known as the berkeley algorithm., 1. allow accurate utc synchronization, 2.aspect invloves synchronizing the concept of physical time throughout the distributed system., 10.4.2 logical ordering -does not rely on a common clock, which cannot exist in a distributed system nor does it rely on synchronized physical clocks., 10.3.4 simple network time protocol (sntp) -sntp is an adaptation of ntp and is employed when ultimate performance is not required. -sntp can operate in all modes available and specified in ntp., 10.3.2 ntp design goals, 10.4 logical clocks, 10.2.1 obtaining an accurate physical time -a physical time server is required to access the current time from a universal time coordinator(utc)., 10.4.3 total ordering with logical clocks -total ordering- no two events can be allowed to occur at precisely the same time., 10.3 network time protocol (ntp) -is designed to operate in a large distributed system and can accommodate systems cnnected via show modem links to the fastest connections possible., 10.3.3 ntp synchronization modes -can synchronize in any of three modes multicast,client server and symmetric mode., 4. protect against malicious or accidental interference. , broadcast based -in broadcast-based implementations for a centralized approach to physical time service, the time service periodically broadcasts the current time to members of the distributed system., 10 distributed synchronization -requires either global time or global ordering., 10.2.3 centralized physical time services -a centralized physical time serve in a distributed operating system may be designed to function in one of two manners.