Robust synchronization of software clocks across the internet

This paper describes algorithms to discipline a computer clock to a source of standard time, such as a GPS receiver or another computer synchronized to such a source. The algorithms are designed for use in the Net- work Time Protocol (NTP), which is used to synchronize computer clocks in the global Internet. They have been incorporated in the NTP software for Unix and Windows and, for the highest accuracy, in the operating system kernels for Sun, DEC and HP workstations. RMS errors on LANs are usually less than 10 μs and on global Internet paths usually less than 5 ms. However, rare disruptions of one kind or another can cause error spikes up to 100 μs on LANs and 100 ms on Internet paths.

IEEE/ACM Transactions on Networking, 2009

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.

IEEE Transactions on Communications, 1991

This paper describes the Network Time Protocol (NTP), which is designed to distribute time information in a large, diverse internet system operating at speeds from mundane to lightwave. It uses a symmetric architecture in which a distributed subnet of time servers operating in a self-organizing, hierarchical configuration synchronizes local clocks within the subnet and to national time standards via wire, radio or calibrated atomic clock. The servers can also redistribute time information within a network via local routing algorithms and time daemons. This paper also discusses the architecture, protocol and algorithms, which were developed over several years of implementation refinement and resulted in the designation of NTP as an Internet Standard protocol. The NTP synchronization system, which has been in regular operation in the Internet for the last several years, is described along with performance data which shows that timekeeping accuracy throughout most portions of the Internet can be ordinarily maintained to within a few milliseconds, even in cases of failure or disruption of clocks, time servers or networks.

Real-Time Systems, 1997

In large-scale systems, such as Internet-based distributed systems, classical clocksynchronization solutions become impractical or poorly performing, due to the number of nodes and/or the distance. We present a global time service for world-wide systems, based on an innovative clock synchronization scheme, dubbed CesiumSpray . The service exhibits high precision and accuracy; it is virtually inde nitely scaleable; and it is fault-tolerant. It is deterministic for real-time machinery in the local area, which makes it particularly well-suited for, though not limited to, large-scale real-time systems. The clock synchronization scheme is a pseudo-hierarchical mix of external and internal synchronization. The root of the hierarchy are the GPS satellites, which \spray" their reference time over a set of nodes provided with GPS receivers, one per local network, where the second level of the hierarchy performs internal synchronization, further \spraying" the external time inside the local network. The algorithm of the second level is inspired on the high precision a posteriori agreement synchronization algorithm, modi ed to follow an external clock, and able to use simple group communication and membership facilities.

1997

In large-scale systems, such a s I n ternet-based distributed systems, classical clocksynchronization solutions become impractical or poorly performing, due to the number of nodes and/or the distance. We p r e s e n t a global time service for worldwide systems, based on an innovative clock synchronization scheme, dubbed CesiumSpray. The service exhibits high precision and accuracy it is virtually inde nitely scaleable and it is fault-tolerant. It is deterministic for real-time machinery in the local area, which m a k es it particularly w ell-suited for, though not limited to, large-scale real-time systems. The clock synchronization scheme is a pseudo-hierarchical mix of external and internal synchronization. The root of the hierarchy are the GPS satellites, which \spray" their reference time over a set of nodes provided with GPS receivers, one per local network, where the second level of the hierarchy performs internal synchronization, further \spraying" the external time inside the local network. The algorithm of the second level is inspired on the high precision a posteriori agreement s y n c hronization algorithm, modi ed to follow an external clock, and able to use simple group communication and membership facilities.

This article presents an overview and some evaluation results of our PSynUTC 1 prototype system for GPS time distribution and time synchronization in Ethernetbased LANs. PSynUTC does not need dedicated GPS receivers at every computing node but uses ordinary data packets for disseminating time information. High accuracy is achieved by combining (1) hardware packet timestamping at the network interface of nodes and switches, (2) high-resolution, high-frequency adder-based clocks with superior adjustment capabilities, and (3) clock rate synchronization algorithms compensating typical TCXO drifts. Our technology can be employed with any network controller chipset that supports the media-independent interface (MII) standard. Moreover, standard device drivers and protocol stacks can be used without any change. Our evaluation results show that PSynUTC will achieve a worst case synchronization accuracy in the 100 ns range, which is an improvement of at least 4 orders of magnitude over conventional software-based approaches like NTP.

2008

Maintaining synchronization of clocks between wireless systems is a well known problem of which significant research has been performed. This has lead to a variety of methods introduced to maintain clock synchronization. Typically, works are heavy weight in that they require constant communication between systems in order to maintain clock synchronization on the order of microseconds. Unfortunately, for many applications the cost of constant communication to ensure clock synchronization is neither desirable nor acceptable. Additionally, for applications such as link state routing, delay measurements and quality of service measurements, clock synchronization on the order of microseconds is not necessary, and synchronization on the order of milliseconds is sufficient. Thus, in this work we present a light weight technique for correcting for clock drift between systems that will allow for millisecond accuracy during long periods of time while requiring no special hardware nor constant communication between systems. Experimental studies of the measured delay between two systems are performed, showing that with a training period, clocks can remained synchronized within a few milliseconds over long periods of time.

The Network Time Protocol, an over 2-decade old and always improving algorithm for synchronizing networked computer clocks, still finds problems for its efficient operation. Many applications need a trustable time system to function correctly (e.g., banking and distributed database servers). With the advent of Quality of Service in computer networks, this problem can be elegantly approached and solved. This article suggests a framework for dealing with clock synchronization on DiffServ Domains, introduces a novel treatment to packets and validates this proposal on a case-study done with live application metrics in a network emulation environment.

2008

In many distributed and pervasive systems the clocks of nodes are required to be synchronized to a unique global time. Due to unpredictable system and environment characteristics, the distance of a local clock from global time is a variable factor very hard to predict. Systems usually adopt measures to guarantee an upper bound on such distance from global time that are very often quite far from typical execution scenarios and thus are of practical little use. This paper describes the Reliable and Self-Aware Clock (R&SAClock), a low-intrusive software service that is able to compute a conservative estimation of distance from an external global time. R&SAClock acts as a new clock that couples information gained from synchronization mechanisms with information collected from the local clock to provide both current time and a self-adaptive reliable estimation of distance from global time. This paper describes the main functions, services and time-related mechanisms of R&SAClock and an implementation for the NTP synchronization mechanism and Linux OS.

ACM SIGCOMM Computer Communication Review, 1991

This paper summarizes issues in computer network timekeeping with respect to the Network Time Protocol, which is used to synchronize time in many of the hosts and gateways of the Internet. It describes the methods used to coordinate and disseminate international time services and how they are incorporated into NTP time servers. It discusses the hazards on reckoning NTP dates with the conventional civil calendar and a standard method for numbering the days. Finally, the paper describes the NTP timescale, its mapping to conventional civil time and date and its interpretation of leap seconds.