Mills, D.L. DCN Local-Network Protocols. DARPA Network Working Group Report RFC-891, M/A-COM Linkabit, December 1983.  Home/Search   Document Not in Database   Summary   Related Articles   Check

....definition implies that the medium has access to a physical clock that can be used to measure the progress of time. In the scope of this work we assume that the physical clocks of all the participants are reasonably synchronized typically with a deviation of less than 50ms (achievable using NTP [68] or GPS clocks) Furthermore we require that the correction of clock drift is done in a way that does not result in decreasing the value of a physical clock, e.g. it can be done by slowing down halting the physical clock for a period of time instead of simply decreasing its value. If a given ....

....to a physical clock are usable for the timing in continuous distributed interactive media since an event or state for this class of media is only valid at a single point in (real) time. This requires the existence of a common physical time base that can be established using protocols like NTP [68] (typical deviation from the real time: less than 50ms) and or GPS receivers (deviation of less then 1ms) If a message arrives late in a continuous medium it cannot be directly applied to the local copy of the medium s state. Instead actions need to be taken to repair the problem. A physical ....

D. L. Mills. Network Time Protocol (Version 3) specification, implementation and analysis. DARPA Network Working Group Report RFC-1305, University of Delaware, 1992.

....that may be vital to the overall behavior of the DVE. We call this piece of information an event. Events carry a timestamp of the time they took place. This assumes that a common time base is available for all participants. This assumption is common for almost all DVEs; they typically use NTP [5] or the GPS timer. While such a common time base may not be sufficient to establish a full ordering on events (e.g. several events may have the same timestamp) this does not pose a problem. Events with the same timestamp are either interpreted to be simultaneous or an additional criterion is used ....

D. L. Mills. Network Time Protocol (Version 3) specification, implementation and analysis. DARPA Network Working Group Report RFC-1305, University of Delaware, 1992.

....to take into account the point in time at which an operation should be executed. In order to give a consistency criterion we de ne a total ordering relation based on physical clocks. We assume that the physical clocks of all application instances are reasonably synchronized 1 (using e.g. NTP [Mil92] or GPS clocks) 1 A complete synchronization of all physical clocks is not necessary since di ering clocks do not endanger the consistency criterion de ned below. However, unsynchronized clocks increase the probability of short term inconsistencies and reduce the fairness among participants. ....

D.L. Mills. Network Time Protocol (Version 3) speci cation, implementation and analysis. DARPA Network Working Group Report RFC-1305, University of Delaware, 1992.

....state may continuously update not only according to the operations issued on it, but also because of the passage of time as well. In our model, it is assumed that the local clocks of all the sites are synchronized periodically . The clock synchronization can be guaranteed by Network time Protocol [23] or GPS clocks, and the clock deviation is less than 50ms. The work reported in [21] has the similar assumption. As shown in Figure 1, we illustrate our example in a 2 dimension environment. It is straightforward to extend this example into 3 D environme nt. Suppose the initial position of object ....

D.L. Mills. Network Time Protocol (Version 3) specificcation, implementation and analysis. DARPA Network Working Group Report RFC-1305, University of Delaware, 1992.

....in the system. Kopetz and Ochsenreiter [5] estimate that it is possible to achieve a clock synchronization tightness in the order of 125 s by implementing the clock synchronization algorithm in the kernel of the operating system. 6. 2 Network Time Protocol (NTP) The Network Time Protocol (NTP) [15] was designed to distribute time information in a large, diverse internet system operating at speeds from mundane to lightwave. Synchronization is achieved via a connected network formed by primary and secondary time servers, clients and interconnecting transmission paths. A primary time server is ....

D. L. Mills, Network Time Protocol (Version 3) Specification, Implementation and Analysis, DARPA Network Working Group Report RFC-1305, University of Delaware, March 1992.

No context found.

Mills, D.L. DCN Local-Network Protocols. DARPA Network Working Group Report RFC-891, M/A-COM Linkabit, December 1983.

No context found.

Mills, D.L. Internet Delay Experiments. DARPA Network Working Group Report RFC-889, M/A-COM Linkabit, December 1983.

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Mills, D.L. DCNET Internet Clock Service. DARPA Network Working Group Report RFC-778, COMSAT Laboratories, April 1981.

No context found.

Mills, D.L. Network Time Protocol (NTP). DARPA Network Working Group Report RFC-958, M/A-COM Linkabit, September 1985.

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Mills, D.L. Algorithms for Synchronizing Network Clocks. DARPA Network Working Group Report RFC-956, M/A-COM Linkabit, September 1985.

No context found.

Mills, D.L. DCN Local-Network Protocols. DARPA Network Working Group Report RFC-891, M/A-COM Linkabit, December 1983.

No context found.

Mills, D.L. Internet Delay Experiments. DARPA Network Working Group Report RFC-889, M/A-COM Linkabit, December 1983.

No context found.

Mills, D.L. DCNET Internet Clock Service. DARPA Network Working Group Report RFC-778, COMSAT Laboratories, April 1981.

No context found.

Mills, D.L. Algorithms for Synchronizing Network Clocks. DARPA Network Working Group Report RFC-956, M/A-COM Linkabit, September 1985.

No context found.

Mills, D.L. DCN Local-Network Protocols. DARPA Network Working Group Report RFC-891, M/A-COM Linkabit, December 1983.

No context found.

Mills, D.L. Internet Delay Experiments. DARPA Network Working Group Report RFC-889, M/A-COM Linkabit, December 1983.

No context found.

Mills, D.L. DCNET Internet Clock Service. DARPA Network Working Group Report RFC-778, COMSAT Laboratories, April 1981.

....that refer to a physical clock are usable for the timing in continuous distributed interactive media, since an event or state for this class of media is only valid at a single point in (real) time. This requires the existence of a common physical time base, established using protocols like NTP [11] (typical deviation from the real time: less than 50ms) and or GPS receivers (deviation of less then 1ms) If a message arrives late in a continuous medium, it cannot be directly applied to the local copy of the medium s state. Instead there need to be taken actions to repair the problem, before ....

D. L. Mills, \Network Time Protocol (Version 3) specication, implementation and analysis," DARPA Network Working Group Report RFC-1305, University of Delaware, 1992.

....a particular peer is determined by a sumof weighted offset differences, called the dispersion. The total dispersion to the root due to all causes is called the synchronization dispersion. 3. Network Time Protocol The Network Time Protocol (NTP) now established as an Internet Standard protocol [22], is used to organize and maintain a set of time servers and transmission paths as a synchronization subnet. NTP is built on the Internet Protocol (IP) 8] and User Datagram Protocol (UDP) 27] which provide a connectionless transport mechanism; however, it is readily adaptable to other protocol ....

....automatically to use backup paths, with the result that one of the servers has dropped from stratum 2 to stratum 3. The following subsections contain an overview of the data formats, entities, state variables andproceduresused in NTP. Further details are contained in the formal specification [22]. The specification is based on the imple1 22 3 33 1 23 333 (a) b) x Figure 1. Subnet Synchronization 4 mentation model illustrated below, but it is not intended that this model be the only one upon which a specification can be based. In particular, the specification is intended to illustrate ....

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Mills, D.L. Network Time Protocol (Version 2) specification and implementation. DARPA Network Working Group Report RFC-1119,University of Delaware, September 1989.

....and received message orders are unimportant and that reliable delivery is not required. Obviously, the accuracies achievable depend upon the statistical properties of the outbound and inbound data paths. Further analysis and experimental results bearing on this issue can be found below and in [5] [19] and [20] As shown in Figure 2, the computed delays and offsets are processed in the data filters to reduce incidental timing noise and the most accurate and reliable subset determined by the peer selection algorithm. The resulting offsets of this subset are first combined on a weighted average ....

.... a particular clock can be trusted is an interesting abstract problem which can be attacked using methods such as described in [14] 15] 18] and [31] A number of algorithms for filtering, smoothing and classifying timekeeping data have been described in the literature [1] 6] 12] 13] [19], including convergence algorithms, which attempt 8 to reduce errors by repeatedly casting out statistical outlyers, and consistency algorithms, which attempt to classify subsets of clocks as trusted or not by comparing statistics such as mean and variance. The NTP data filtering algorithm, ....

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Mills, D.L. Algorithms for synchronizing network clocks. DARPA Network Working Group Report RFC-956, M/A-COM Linkabit, September 1985.

....and unreliable networks. This section contains an overview of the architecture and algorithms used in NTP. A detailed description of the architecture and service model is contained in [MIL91] while the current protocol formal specification, designated NTP Version 3, is defined in RFC 1305 [MIL92a]. A subset of the protocol, designated Simple Network Time Protocol (SNTP) is presented in RFC1361 [MIL92c] A security analysis of NTP is presented in [BIS90] NTP and its implementations have evolved and proliferated in the Internet over the last decade, with NTP Version 2 adopted as an ....

....from one to 17 minutes are used to determine individual roundtrip delays and clock offsets, as well as provide reliable error bounds. Asshown in the figure, the computed delays and offsets for each peer are processed by the clock filter algorithm to reduce incidental jitter. As described in [MIL92a], this algorithm selects from among the last several samples the one with minimum delay and presents the associated offset as the output. The clock selection algorithm determines from among all peers a suitable subset capable of providing the most accurate and trustworthy time using principles ....

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Mills, D.L. Network Time Protocol (Version 3) specification, implementation and analysis. DARPA Network Working Group Report RFC-1305, University of Delaware, March 1992, 113 pp.

.... designated Simple Network Time Protocol (SNTP) is presented in RFC1361 [MIL92c] A security analysis of NTP is presented in [BIS90] NTP and its implementations have evolved and proliferated in the Internet over the last decade, with NTP Version 2 adopted as an Internet Standard (Recommended) [MIL89] and its successor NTP Version 3 adopted as a Internet Standard (Draft) NTP is built on the Internet Protocol (IP) DAR81a] and User Datagram Protocol (UDP) POS80] which provide a connectionless transport mechanism; however, it is readily adaptable to other protocol suites. The protocol can ....

Mills, D.L. Network Time Protocol (version 2) - specification and implementation. DARPA Network Working Group Report RFC-1119, University of Delaware, September 1989.

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Mills, D.L. Internet Delay Experiments. DARPA Network Working Group Report RFC-889, M/ACOM Linkabit, December 1983.

....in a virtualized PDP11 architecture is painfully slow, so the DCN system remained a demonstration curio. E 116 By 1977 the DCN system was dismantled and rebuilt in a leaner, zippier design and software was developed for the TCP IP protocol suite. The DCN routing protocol, now called Hellospeak [12], was completely redesigned to link DCN hosts and gateways together and to other networks, including ARPANET, SATNET and various LANs. At the time there were no full featured Internet hosts other than DCN based on a microprocessor; so, presumably because nobody knew what DCN stood for, they became ....

....basis, including the retransmission timeouts, packetization delays and acknowledgement delays. A particularly troublesome issue was the determination of retransmission timeout under conditions of moderate segment loss and path delays varying over several orders of magnitude. An extensive survey [12], conducted with the aid of Fuzzballs used as network sounders, revealed the Internet had path delays and delay dispersions much larger than imagined. E 113 As the result of these experiments, several modifications to the suggested TCP retransmission timeout estimation algorithm were made, ....

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Mills, D.L. DCN local-network protocols. DARPA Network Working Group Report RFC-891, M/ACOM Linkabit, December 1983.

....in NTP. A detailed description of the architecture and service model is contained in [MIL91a] while the current protocol specification, designated NTP Version 3, is defined by RFC 1305 [MIL92a] A subset of the protocol, designated Simple Network Time Protocol (SNTP) is described in RFC 1361 [MIL92c]. NTP and its implementations have evolved and proliferated in the Internet over the last decade, with NTP Version 2 adopted as an Internet Standard (Recommended) MIL89] and its successor NTP Version 3 adopted as a Internet Standard (Draft) MIL92a] NTP is built on the Internet Protocol (IP) ....

Mills, D.L. Simple Network Time Protocol (SNTP). DARPA Network Working Group Report RFC-1361, University of Delaware, August 1992, 10 pp.

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