Simons, B., Welch, J.L., Lynch, N.A.: An overview of clock synchronization. Lecture Notes in Computer Science 448 (1990) p.84--96  Home/Search   Document Not in Database   Summary   Related Articles   Check

....a stimulus for the development of algorithm verification techniques. Similar results should be possible for real time systems. Some examples of complexity results that have already been obtained for real time systems are the many results on clock synchronization, including [8, 11, 17, 20, 32] see [31] for a survey) In this paper, we embark on a study of complexity results for real time systems. We begin this study by considering timing based variations of certain problems that have previously been studied in asynchronous concurrent systems. In particular, we study a variant of the mutual ....

SIMONS, B., WELCH, J. L., AND LSCH, N. (1988), "An Overview of Clock Synchronization, " IBM Technical Report RJ 6505.

....However, with the introduction of timers we can make the optimistic assumption that the timer can be trusted for synchronization. This implies that not all modules need to communicate at each time step to stay synchronized. We can use one of the many algorithms for synchronizing clocks (see [11] [16] for an overview) and achieve increased efficiency because the local timer can be relied on for deciding when to perform a new action. However this is not the path we will follow here. In order to achieve coordinated global behavior of the robot it is important that modules act at the right time ....

B. Simons, J.L. Welch, and N. Lynch. An overview of clock synchronization. In Asilomar Workshop on Fault-tolerant Distributed Computing Conf., volume 448 of Lecture Notes in Computer Science, pages 84--96, 1990.

....any key of the one way key chain commits to all following keys, so we call such a key a one way key chain commitment, or simply key chain commitment. 2. 2 Time Synchronization TESLA does not need the strong time synchronization properties that sophisticated time synchronization protocols provide [22, 24, 37], but only requires loose time synchronization, and that the receiver knows an upper bound on the sender s local time. We now outline a simple and secure time synchronization protocol that achieves this requirement. For simplicity, we assume the clock drift of both sender and receiver is ....

....3.3 Bootstrapping Receivers Before a receiver can authenticate messages with TESLA, it needs to be loosely time synchronized with the sender, know the disclosure schedule of keys, and receive an authenticated key of the one way key chain. Various approaches exist for time synchronization [24, 37, 22]. TESLA, however, only requires loose time synchronization between the sender and the receivers, so a simple algorithm is sufficient. The time synchronization property that TESLA requires is that each receiver can place an upper bound of the sender s local time, as we discuss in Section 2.2. The ....

B. Simons, J. Lundelius-Welch, and N. Lynch. An overview of clock synchronization. In B. Simons and A. Spector, editors, Fault-Tolerant Distributed Computing, number 448 in LNCS, pages 84--96, 1990.

....sound, light, air pressure) is sensed by di erent smart things. These examples indicate that temporal ordering and other real time issues play an important role in such environments. As we will see later, neither logical time [12, 14] nor classical physical clock synchronization algorithms [3, 13, 16, 17] can be used to solve this problem in general. We will suggest an algorithm that solves the temporal ordering problem and other real time issues in environments sketched above. 2. AD HOC NETWORKS Ad hoc networks [2] are networks of mobile wireless computing devices. Due to the limited ....

....at the ends. It remains an open task to determine good probability distributions. Furthermore it has to be investigated for which cases knowing a probability instead of MAYBE is advantageous for applications. 8. RELATED WORK There has been much work on physical clock synchronization in the past [3, 13, 16, 17]. However, most of the proposed synchronization algorithms, including the well known Network Time Protocol [15] rely on a network that is not partitioned and where it is always possible to produce good estimations for the message delay. As pointed out in section 3, this is not the case for sparse ....

B. Simons, J. Welch, and N. Lynch. An overview of clock synchronization. Technical Report RJ 6505, IBM Almaden Research Center, 1988.

....synchronization, that deals with both drift and consonance embedded in a realistic system model. Furthermore, we work out the similarities to the well known problem of clock state synchronization, in order to benefit from it s rich collection of theoretical results and approaches, cf. Sch87] or [SWL90]. The paper is organized as follows: Section 2 introduces formally the clock rate synchronization problem and stipulates our system model concerning clocks and processors along with their means of communication. After a comprehensive preparatory work on notations and building blocks for rate ....

B. Simons, J. Lundelius-Welch, N. Lynch. An Overview of Clock Synchronization, in B. Simons, A. Spector (eds.): Fault-Tolerant Distributed Computing, Springer Lecture Notes on Computer Science 448, 1990, p. 84--96.

....which is beyond the scope of this thesis. 1. 2 Previous Work Different variants of the clock synchronization problem have been the target of a vast amount of research from both practical viewpoint (e.g. 26, 6, 24, 28, 1, 15] and theoretical viewpoint (e.g. 16, 19, 7, 13, 33, 3] surveys [31, 30] and references therein) the exact definition of the problem depends both on the intended use of the clocks and on the specific underlying system. The large number of variants is justified by the wide spectrum of applications. One of the popular variants studied theoretically is internal ....

....is required to provide bounds on the length of this real time interval, and the smallest difference in an execution is the internal tightness of that execution. The task of internal synchronization has been the target of considerable research (see, e.g. 19, 7, 13, 3] and the survey [31]) However, to the best of our knowledge, the only known non trivial lower bounds for internal tightness were for the case of drift free clocks. In this chapter, based on synchronization graphs, we give a lower bound for the internal tightness in a synchronization system with bounded drift clocks. ....

B. Simons, J. L. Welch, and N. Lynch. An overview of clock synchronization. Research Report RC 6505 (63306), IBM, 1988.

....a clock knows at any time if it is synchronized or not with the others, whereas in statistical algorithms, clocks do not know how far apart they are from each others. Clock synchronization has been extensively studied for the last twenty years. Thorough surveys can be found in [Sch86, RSB90] and [SWL90]. In [RSB90] software and hardware clock synchronization algorithms are classified with regard to the clock correction scheme used. In contrast, the algorithms surveyed in [SWL90] are listed according to the supported faults and the system synchrony (i.e. knowledge of upper bounds on ....

....has been extensively studied for the last twenty years. Thorough surveys can be found in [Sch86, RSB90] and [SWL90] In [RSB90] software and hardware clock synchronization algorithms are classified with regard to the clock correction scheme used. In contrast, the algorithms surveyed in [SWL90] are listed according to the supported faults and the system synchrony (i.e. knowledge of upper bounds on communication latencies) Schneider s work [Sch86] gives a single unifying paradigm and correctness proof that can be used to understand mostly all deterministic clock synchronization ....

B. Simons, J. Lundelius Welch, and N. Lynch. An overview of clock synchronization. In Spector, editor, Asilomar Workshop on Fault-tolerant Distributed Computing Conference, volume 448, pages 84--96. Lecture Notes in Computer Science, 1990.

....merit a book of its own. 20.1 Clock Synchronization Clock synchronization is an example of a topic that until the recent past represented an important area for distributed systems research [LAM84, LM85, Mar84, LM85, KO87, ST87, Cri89, CF94, VR92, CM96] overviews of the field can be found in [SWL90] and [Lis93] The introduction of the global positioning system, in the early 1990 s, greatly changed the situation. As recently as five years ago, a textbook such as this would have treated the problem in considerable detail, to the benefit of the reader because the topic is an elegant one and ....

....the communication overhead required in support of this periodic model, integrating real time communication with other periodic or real time actions, priority inversion in communication environments, and other topics in the area. 20.4 Related Readings On clock synchronization see the review in [SWL90], other references include [Lam84, Mar84, LM85, KO87, ST87, Cri89] On the a posteriori method: VR92, CM96] On the CASD protocol: CASD85, CSDA90, CS95, Cri96, CSTC90] On the MARS system: KO87, DRSK89, KV93] On Delta 4: Pow91, Pow94, RV89, RVR93, Ver93, Ver94] On real time work with Horus: ....

Barbara Simons, Jennifer N. Welch, and Nancy Lynch. An Overview of Clock Synchronization. In Fault-Tolerant Distributed Computing (Simons and Spector, eds), Springer Verlag Lecture Notes in Computer Science 448, 1990, 8496.

....algorithms, see [Lis93] for some examples. Providing mutually synchronized local clocks is known as the internal clock synchronization problem, and numerous solutions have been worked out at least in scientific research under the term fault tolerant clock synchronization, see [RSB90] [SWL90] for an overview and [YM93] for a bibliography. If system time provided by synchronized clocks must also have a well defined relation to Universal Time Coordinated (UTC) the only official and legal standard time, then the fault tolerant external clock synchronization problem needs to be ....

B. Simons, J. Lundelius-Welch, N. Lynch. An Overview of Clock Synchronization, in B. Simons, A. Spector (eds.): Fault-Tolerant Distributed Computing, Springer Lecture Notes on Computer Science 448, p. 84--96, 1990.

....such as causal relationships [Lam78] In addition, we do not detail issues speci c to clock synchronization in large scale distributed systems (e.g. SR87, VCR97] Clock synchronization has been extensively studied for the last twenty years. Thorough surveys can be found in [Sch86, RSB90] and [SWL90]. In [RSB90] software and hardware clock synchronization algorithms are classi ed with regard to the clock correction scheme used. In contrast, the algorithms surveyed in [SWL90] are listed according to the supported faults and the system synchrony (knowledge of upper bounds on communication ....

....has been extensively studied for the last twenty years. Thorough surveys can be found in [Sch86, RSB90] and [SWL90] In [RSB90] software and hardware clock synchronization algorithms are classi ed with regard to the clock correction scheme used. In contrast, the algorithms surveyed in [SWL90] are listed according to the supported faults and the system synchrony (knowledge of upper bounds on communication latencies) Schneider s work [Sch86] gives a single unifying paradigm and correctness proof that can be used to understand mostly all deterministic clock synchronization algorithms ....

B. Simons, J. Lundelius Welch, and N. Lynch. An overview of clock synchronization. In Spector, editor, Asilomar Workshop on Fault-tolerant Distributed Computing Conference, volume 448, pages 8496. Lecture Notes in Computer Science, 1990.

....al. 8] use similar techniques to obtain synchronization algorithms for relativistic systems. Much effort has been devoted to studying internal synchronization, where the goal is to synchronize clocks within a system in which real time is not available (see, e.g. 11, 12, 6, 10, 24, 1] surveys [22, 21] and references therein) The approach of comparing the synchronization bounds to the best possible bound for the given execution was first presented by Attiya et al. in [1] where they studied internal synchronization. The work in [1] extended the work of Halpern et al. 10] which analyzed ....

B. Simons, J. L. Welch, and N. Lynch. An overview of clock synchronization. Research Report RC 6505 (63306), IBM, 1988.

....our algorithms may be run without modification (with c 1 = c 2 ) in the model of [15] yielding the running times derived here with the syntactic substitution of ffi for d and Delta=ffi for C. Other work in this area includes the extensive literature on clock synchronization algorithms (see [20] for a survey) Algorithms from [22] and the previously mentioned consensus work of [11] include ideas similar to those in our simulation tolerant of arbitrary failures. Other problems recently studied in our model of timing uncertainty include the problem of mutual exclusion [2] and the ....

B. Simons, J. L. Welch and N. Lynch. An overview of clock synchronization. Proceedings of IBM Fault-Tolerant Computing Workshop, March, 1986.

....its own local processor clock, processors do not have a common notion of time. In practice, as stated in [3] no two processor clocks run at exactly the same rate, as a result of which the clocks will drift away from each other. Several fault tolerant clock synchronization algorithms exist (see [7] for an overview) by means of which the processor clocks can be synchronized within certain bounds. Unfortunately, in practice, the difference in synchronization between any two processor clocks will never be zero. This makes it hard to have all correct processors agree on a common point in time. ....

....communication network with perfect communication links. This 1 1 r c t 1 c t 2 t 1 t 2 1 r ( assumption is justified by the fact that we can model a link failure as a failure of one of its adjacent processors [7]. A8. A correct processor rejects all faulty messages it receives. A9. Correct messages received in time in a correct processor c are accepted by c. A10. Correct messages received too late in a correct processor c are rejected by c. A11. Any correct message that is timely received by a correct ....

Simons, B., Lundelius Welch, J., and Lynch, N., An Overview of Clock Synchronization, in: Simons, B., and Spector, A. (Eds.), Fault-Tolerant Distributed Computing, Springer Verlag, Berlin, 1990, pp.84-96.

....depending on the item x and on the current time, in much the same way as the user ID is used in protocol ACP 0 . Note that the servers must all use the same time value otherwise the key cannot be reconstructed. However, maintaining synchronized clocks is a non trivial and costly task (cf. [SWL90]) which we prefer to avoid. Instead, in our modified protocol ACP 00 (see Figure 3) the user attaches a timestamp t to the request for item x, which is then used by the servers in the share generation. This might allow a cheating user to use fake timestamps, so on receiving such a request, the ....

B. Simons, J. L. Welch, and N. Lynch. An overview of clock synchronization. In B. Simons and A. Spector, editors, Fault-Tolerant Distrib. Comp., LNCS 448. Springer-Verlag, 1990.

....depending on the item x and on the current time, in much the same way as the user ID is used in protocol ACP 0 . Note that the servers must all use the same time value otherwise the key cannot be reconstructed. However, maintaining synchronized clocks is a non trivial and costly task (cf. [45]) which we prefer to avoid. Instead, in our modified protocol ACP 00 (see Figure 3) the user attaches a timestamp t to the request for item x, which is then used by the servers in the share generation. This might allow a cheating user to use fake timestamps, so on receiving such a request, the ....

B. Simons, J. L. Welch, and N. Lynch. An overview of clock synchronization. In B. Simons and A. Spector, editors, Fault-Tolerant Distrib. Comp., LNCS 448. Springer-Verlag, 1990.

....are realistic for a particular network to the problem of verifying one observable property of the network. Thus our methods have application far beyond the proof of correctness of any particular algorithm. Although a lot of work has been done already in the area of clock synchronization [SWL90, YM93], the kinds of questions addressed in this paper are new. Lower and upper bounds for synchronizing by broadcasting given in [HS91] do not apply here because that paper assumes that broadcast messages may have arbitrary delay, but all messages of each single broadcast arrive at the recipients ....

B. Simons, J. Welch, and N. Lynch, An Overview of Clock Synchronization, in B. Simons and A. Spector eds., Fault-Tolerant Distributed Computing, LNCS 448, Springer-Verlag, 84-96, 1990.

.... O(x K ) for all x 0. although covered by the system model above is very unlikely in practice. 3. CLOCK VALIDATION TECHNIQUES Unlike in the context of (internal) clock synchronization, where much work has been done (see (Yang and Marsland, 1993) for a comprehensive bibliography and (Simons et al. 1990; Ramanathan et al. 1990) for overviews) there are only a few papers devoted to the problem of external synchronization. The latter research may be categorized as follows: In (Kopetz and Ochsenreiter, 1987) some helpful general considerations concerning external synchronization and an outline ....

Simons, B., Lundelius-Welch, N., and Lynch, N. (1990). An Overview of Clock Synchronization.

....the absence of a message may be much more costly than receiving the message. Our algorithms run equally well in this model; we remark on how our bounds translate to this model in Section 6.1. Other work in this area includes the extensive literature on clock synchronization algorithms (see [SWL86] for a survey) Other problems recently studied in our model of timing uncertainty include the problem of mutual exclusion ( AL89] and the complexity of a network synchronizer algorithm ( AM90] 1.3 Results of this thesis 1.3.1 Consensus in the presence of omission failures In Chapter 3, we ....

B. Simons, J. L. Welch and N. Lynch. An overview of clock synchronization. Proceedings of IBM Fault-Tolerant Computing Workshop, March, 1986.

....synchronization is required to provide bounds on the length of this real time intervalEand the smallest difference in an execution is the iteval tighte of that execution. The task of internal synchronization has been the target of considerable research (seeE e.g.E[19E7E13E3] and the survey [31]) HoweverEto the best of our knowledgeEthe only known non trivial lower bounds for internal tightness were for the case of drift free clocks. In this chapterEbased on synchronization graphsEwe give a lower bound for the internal tightness in a synchronization system with bounded drift clocks. We ....

B. Simons1`J. L. Welchi`and N. Lynch. An overview of clock synchronization. Research Report RC 6505 (63306)1`IBM1`1988.

....round of computation to solve the consensus problem or the renaming problem under timing failures in the semi synchronous model in the presence of the timing failure. Other works developing efficient algorithms for various problems in different timing models include clock synchronization (see [SWL88] for survey) leader election [CT90] renaming [AD93] transaction commit [CW90] and atomic broadcast [CASD86, CDS90] 1.7 Our Results 1.7.1 Known Unknown Bound Models Tables 2 and 3 summarize our results on the lower and upper bounds for the time complexity of solving the (s; n) session problem. ....

B. Simons, J. Welch and N. Lynch, "An Overview of Clock Synchronization, " IBM Computer Science Research Report RJ 6506 (63306), Oct. 1988.

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Simons, B., Welch, J.L., Lynch, N.A.: An overview of clock synchronization. Lecture Notes in Computer Science 448 (1990) p.84--96

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B. Simons, J. Welch, and N. Lynch. An overview of clock synchronization. In Fault-Tolerant Distributed Computing, volume 448 of LNCS, pages 84--96, 1990.

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B. Simons, J. L. Welch, and N. Lynch. An overview of clock synchronization. In Fault-Tolerant Distributed Computing, volume 448 of LNCS, pages 84--96, 1990.

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Barbara Simons, Jennifer Lundelius-Welch, and Nancy Lynch. An overview of clock synchronization. In Barbara Simons and A. Spector, editors, FaultTolerant Distributed Computing, pages 84--96. Springer Verlag, 1990. (Lecture Notes on Computer Science 448).

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Simons B., Lundelius-Welch J. and Lynch N., "An Overview of Clock Synchronization", in Simons B. and Spector A., (eds.): Fault-Tolerant Distributed Computing, Springer Lecture Notes on Computer Science 448, pp. 84--96, 1990.

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