António Casimiro

xAMp Time Service Specification

Luís Rodrigues, Paulo Veríssimo and António Casimiro

Technical Report, RT/66-91, Delta-4 Project, INESC, Lisboa, Portugal, October 1991


The availability of a common time base is a mandatory requirement for many distributed activities such as, between many others, the execution on different nodes of coordinated actions in the time domain, measuring of distributed activities, and timestamping of events. It is possible to provide such a common time base using a centralized time service, resident in a single node of the system. This solution is not fault tolerant and exhibits poor performance if clocks need to be frequently read. The most efficient (and common) solutions for the clock synchronization problem rely on the (processor's) underlying hardware clock to create at each node a virtual clock, which is synchronized with the other virtual clocks by a clock synchronization algorithm. Clock synchronization is a complex issue on his own, thus it cannot be dealt in detail on this guide. The interested reader should consult the bibliography, where xAMp's clock synchronization service is described in detail [Rodrigues:91]. The algorithm is based on a new variant of the well-known convergence non-averaging technique, dubbed a posteriori agreement that exploits the intrinsic characteristics of broadcast networks. The advantage of this approach is that the precision achieved by the algorithm is not limited, as opposite to most of the published works, by the variability or worst case values of network access delays and it does not require the use of dedicated hardware. The clock synchronization service is integrated in the xAMp service. The reader should consult the xAMp chapter of the implementation guide where the remaining services of xAMp are described.


  author       = "Lu\'{i}s Rodrigues and Paulo Ver\'{\i}ssimo and Ant\'{o}nio Casimiro",
  title        = "{xAMp} {T}ime {S}ervice Specification ",
  institution  = "Delta-4 Project, INESC",
  year         = 1991,
  abstractURL  = "",
  url          = "",
  type         = "",
  number       = "RT/66-91",
  address      = "Lisboa, Portugal",
  month        = oct,
  keywords     = ""

Technical report

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