We present a detailed re-examination of the problem of inexpensive yet accurate clock synchronization for networked devices. Based on an empirically validated, parsimonious abstraction of the CPU oscillator as a timing source, accessible via the TSC register in popular PC architectures, we build on the key observation that the measurement of time differences, and absolute time, requires separate clocks, both at a conceptual level and practically, with distinct algorithmic, robustness, and accuracy characteristics. Combined with round-trip time based filtering of network delays between the host and the remote time server, we define robust algorithms for the synchronization of the absolute and difference TSCclocks over a network. We demonstrate the effectiveness of the principles, and algorithms using months of real data collected using multiple servers. We give detailed performance results for a full implementation running live and unsupervised under numerous scenarios, which show very high reliability, and accuracy approaching fundamental limits due to host system noise. Our synchronization algorithms are inherently robust to many factors including packet loss, server outages, route changes, and network congestion.

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54 communications of th e ac m | m ay 2 0 1 0 | vo l . 5 3 | n o. 5 practice eVe rYoNe, a N d Mo s T everything, needs a clock, and computers are no exception. however, clocks tend to ultimately drift off, so it is necessary to bring them to heel periodically through synchronizing to some other reference clock of higher accuracy. An inexpensive and convenient way to do this is over a computer network.Since the early days of the Internet, a system collectively known as nTP (network Time Protocol) has been used to allow client computers, such as PCs, to connect to other computers (nTP servers) that have high-quality clocks installed in them. Through an exchange of packet timestamps transported in nTPformatted packets over the network, the PC can use the server clock to correct its own clock. As the NTP clock software, in particular the ntpd daemon, comes packaged with all major computer operating systems, including Mac OS, Windows, and Linux, it is a remarkably successful technology with a user base on the order of the global computer population. Although the NTP system has operated well for general-purpose use for many years, both its accuracy and robustness are below what is achievable given the underlying hardware, and are inadequate for future challenges. One area where this is true is the telecommunications industry, which is busy replacing mobile base-station synchronous backhaul systems (which used to provide sub-microsecond hardwarebased synchronization as a by-product) with inexpensive asynchronous Ethernet lines. Another is high-speed trading in the finance industry, where a direct relationship exists between reducing latencies between exchanges and trading centers, and the ability to exploit these for profit. Here accurate transaction timestamps are crucial. More generally, timing is of fundamental importance because, since the speed of light is finite, the latencies between network nodes are subject to hard constraints that will not be defeated by tomorrow's faster processors or bandwidth increases. What cannot be circumvented must be tightly managed, and this is impossible without precise synchronization. the clockworkWhen the discussion turns to clocks, confusion is often not far behind. To avoid becoming lost in the clockwork, let's define some terms. By t we mean true time measured in seconds in a Newtonian universe, with the origin at some arbitrary time point. We say that a clock C reads C(t) at true time t. Figure 1 shows what some example clocks read as (true) time goes on. The black clock Cp(t) is perfect: C p (t) = t, whereas the blue clock C s (t) = C 0 + (1 + x)t is out by C 0 when t = 0. In fact it keeps getting worse as it runs at a constant but overly

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