6- logical_time

6- logical_time - CSE 486/586 Distributed Systems Logical...

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CSE 486/586, Spring 2012 CSE 486/586 Distributed Systems Logical Time Steve Ko Computer Sciences and Engineering University at Buffalo
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CSE 486/586, Spring 2012 Last Time Clock skews do happen External and internal synchronization Cristian’s algorithm: external synchronization Berkeley algorithm: internal synchronization Both designed for LAN NTP (Network Time Protocol) Hierarchy of time servers Estimates the actual offset between two clocks Designed for the Internet Logical time For ordering events, relative time should suffice. Will continue today 2
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CSE 486/586, Spring 2012 Brief Recap: Cristian’s Algorithm Cristian’s algorithm A client asks its time server. The time server sends its time T . The client estimates the one-way delay and sets its time. » It uses T + RTT/2 The correct time can be between [T + min, T + RTT – min] Min one-way delay: min, max one-way delay: RTT – min The accuracy is: +-(RTT/2 – min) Natural next step: minimize inaccuracy Take multiple readings and use the minimum RTT  tighter bound For unusually long RTTs, ignore them and repeat the request  removing outliers 3
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CSE 486/586, Spring 2012 Brief Recap: NTP The Internet Arbitrary delays; hard to estimate one-way delay Too many hosts; cannot rely on a few time servers Thus, hard to apply Cristian’s algorithm The NTP Doesn’t estimate one-way delay; instead estimate the actual offset between two clocks Uses a hierarchy of servers  scales better How to estimate the actual offset 4
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CSE 486/586, Spring 2012 Theoretical Base for NTP 5 T i T i-1 T
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6- logical_time - CSE 486/586 Distributed Systems Logical...

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