Network Time Protocol(NTP)

Network Time Protocol(NTP) tài liệu, giáo án, bài giảng , luận văn, luận án, đồ án, bài tập lớn về tất cả các lĩnh vực k...

KRONE facts KRONE (Australia) Holdings Pty Limited 2 Hereford Street Berkeley Vale NSW 2261 PO Box 335 Wyong NSW 2259 Phone: 02 4389 5000 Fax: 02 4388 4499 Tech Support: 1800 801 298 Email: kronehlp@krone.com.au Web: krone.com.au Copyright © 2004 KRONE (Australia) Holdings Pty. Limited Job No.: 6229 09/04 The Patch By Exception Solution How to save time and money while improving performance and aesthetics In the race for high performance, it is important to consider the elements that go beyond pure performance. KRONE's HIGHBAND ® wire termination module is a unique product in the world of security, alarms and voice and data communications infrastructure. HIGHBAND modules not only provide industry- leading performance, but also many additional benefits that make installation, administration and maintenance of a structured cabling system fast and efficient. Ultimately leading to big cost savings over the lifetime of the asset. THE NEED One common method of achieving communications management at the central termination point for the floor (floor distributor) is to use 8-way modular outlets (RJ45 style) in patch panels. This has the very desirable advantage that anyone authorised in the organisation can change the position of the patch cords to suit the needs at the time and direct signals to different parts of the office. You do not need to be a licensed technician to change the location of the patch cords, but you do need permission from the IT system administrator. The downside is, that in a very short period of time, those minor patch cord relocations leave the patch panel looking extremely cluttered, unsightly and impossible to manage or work on. The result is that incorrect circuits are disconnected causing considerable loss of time and productivity, not to mention user frustration. For those system administrators or technicians doing fault finding on patch panel systems, there is an additional complication. RJ45 style outlets do not allow you to monitor what is happening, so to test the system you have to disconnect the patch cord circuit. Imagine a doctor having to remove an organ just to see if it was causing the problem. Patch cord mis-management and an inability to test/monitor circuits are a costly operational nightmare. Well, there is a better way…one that will not result in a tangled mess of patch cords… IMMEDIATE COST BENEFITS Cost Savings in Jumper Cable The first benefit starts even before the system is activated. The 4-pair jumper cables connecting the first and second verticals can actually be off-cuts from the horizontal cable used in the rest of the building. Whilst utilising off-cuts, there is no need to purchase extra cable or special jumper cable. Cost Savings in Patch Cords You'll also save on the purchase of patch cords for every connection in the floor distributor, because it has already been hard wired with jumpers. The system will run correctly from day one, as is. Figure 1: HIGHBAND 25 modules used in a Patch By Exception arrangement. KRONE facts What To Do Use disconnection modules instead of patch panels. The cabling on the floor distributor inside the telecommunications room is terminated on disconnection modules, either 8-pair or 25-pair. Referring to Figure 2, one vertical column of disconnection modules, the network switch field, is terminated with system leads that plug straight into the front ports of a hub or switch/router with a standard RJ45. All of the work area horizontal cabling from the telecommunications outlets is terminated onto a second vertical column of disconnection modules, the work area field, which is located beside the first vertical column. The technicians simply terminate a series of 4-pair cables known as " jumpered cross-connects" on to the disconnection modules to complete the connection Network Time Protocol(NTP) Network Time Protocol(NTP) Bởi: Wiki Pedia Giao thức NTP (Network Time Protocol - Giao thức đồng thời gian mạng) giao thức để đồng đồng hồ hệ thống máy tính thông qua mạng liệu chuyển mạch gói với độ trễ biến đổi Giao thức thiết kế để tránh ảnh hưởng độ trễ biến đổi cách sử dụng đệm jitter NTP tên gọi phần mềm triển khai dự án Dịch vụ NTP Công cộng (NTP Public Services Project) NTP giao thức Internet lâu đời sử dụng (từ trước năm 1985) NTP thiết kế Dave Mills trường đại học Delaware, ông quản lý với nhóm người tình nguyện NTP liên quan đến giao thức đơn giản DAYTIME (RFC 867 ) TIME (RFC 868 ) Tổng quan NTP sử dụng thuật toán Marzullo, hỗ trợ tính giây nhuận NTPv4 thông thường đảm bảo độ xác khoảng 10 mili giây (1/100 s) mạng Internet công cộng, đạt đến độ xác 200 micro giây (1/5000 s) hay điều kiện lý tưởng môi trường mạng cục Trên mạng Internet, NTP đồng đồng hồ hệ thống máy tính theo UTC; môi trường LAN độc lập, NTP thường sử dụng để đồng với UTC, nguyên tắc sử dụng để đồng với mốc thời gian khác, ví dụ múi chỗ Chi tiết hoạt động NTP quy định RFC 778 , RFC 891 , RFC 956 , FRC 958 (thay 1305), RFC 1305 Chuẩn triển khai phiên (NTPv4 ); nhiên, vào năm 2005, có phiên phiên cũ quy định RFCs Tổ chức IETF NTP Working Group chuẩn hóa hoạt động cộng đồng NTP từ có RFC 1305 1/5 Network Time Protocol(NTP) Một phiên đơn giản NTP không cần yêu cầu lưu trữ thông tin trao đổi cũ gọi Giao thức Đồng Thời gian mạng Đơn giản - Simple Network Time Protocol hay SNTP Giao thức sử dụng cho thiết bị nhúng ứng dụng không cần độ xác cao thời gian Xem RFC 1369 , RFC 1769 , RFC 2030 RFC 4330 Chú ý NTP cung cấp thời gian UTC, thông tin múi hay tiết kiệm ánh sáng ngày (Daylight saving time) Thông tin vày nằm hoạt động NTP xác định cách khác (hầu hết hệ thống cho phép chỉnh thông số này) Triển khai phần mềm NTP Unix Đối với hệ thống UNIX đại, NTP Client triển khai dạng tiến trình daemon chạy liên tục user space Vì tính nhạy cảm với đồng thời gian, cần phải có đồng hồ NTP chuẩn phase-locked loop triển khai kernel space Tất phiên gần Linux, BSD, Solaris áp dụng cách Microsoft Windows Tất phiên Microsoft Windows từ phiên 2000 có Dịch vụ Đồng Windows (Windows Time Service), có chức đồng đồng hồ máy tính với NTP server Tuy nhiên, phiên Windows 2000 triển khai Simple NTP, không tương thích với chuẩn NTP phiên Từ phiên Windows 2003, Microsoft áp dụng phiên đầy đủ NTPv3 theo RFC1305 cho Windows Time Service Tuy nhiên, Windows Time Service đảm bảo độ xác 1-2 giây Microsoft không đảm bảo không hỗ trợ xác dịch vụ W32Time nút mạng Dịch vụ W32Time không hỗ trợ đầy đủ tính mà ứng dụng nhạy cảm với thời gian cần Đồng hồ tham chiếu NTP cài đặt hệ thống Microsoft Windows Thông thường phần mềm miễn phí nhà sản xuất đồng hồ tham chiếu từ GPS cài đặt thông qua Microsoft Installer 2/5 Network Time Protocol(NTP) Clock strata Mũi tên vàng kết nối trực tiếp; mũi tên đỏ kết nối thông qua mạng The U.S Naval Observatory Đồng hồ chủ dự phòng Schriever AFB (Colorado) nguồn Stratum-0 cho NTP NTP sử dụng kiến trúc phân cấp, phân lớp cho cấp nguồn đồng bộ, cấp phân cấp gọi môt "statum' gán số cấp cấp cao Cấp stratum qua trung gian để đến cấp tham chiếu cấp stratum giúp tránh tham chiếu vòng phân cấp Chú ý cấp stratum ý nghĩa chất lượng hay độ ổn định, dễ dàng tim thấy nguồn đồng "stratum 3" có chất lượng tốt nguồn "stratum 2" khác Định nghĩa statum khác với stratum dùng đồng viễn thông 3/5 Network Time Protocol(NTP) Stratum Bao gồm thiết bị đồng hồ nguyên tử (atomic clock), đồng hồ GPS hay đồng hồ vo tuyến khác Thiết bị Stratum-0 thường không kết nối trực tiếp vào mạng mà kết nối với máy tính (ví dụ thông qua cổng RS-232 sử dụng tín hiệu xung) Stratum Đây máy tính kết nối với thiết bị Stratum Đây nguồn đồng hồ tham chiếu cho server Stratum Các máy tính gọi time server Các server Stratum (với NTPv3 hay trước đó) không hoạt động với độ xác cấp Stratum Stratum Là máy tính gửi yêu cầu NTP đến cho server Stratum Thông thường máy tính Stratum tham chiếu từ nhiều server Stratum sử dụng thuật toán NTP để thu thập thông tin xác nhất, bỏ tham chiếu đến server Stratum hoạt động không xác Các máy tính Stratum liên lạc với máy tính Stratum khác để có thời gian xác ổn định nhóm Máy tính Stratum theo phân cấp lại nguồn tham chiếu cho yêu cầu từ Stratum Stratum Các máy tính mày thực chức Stratum 2, tương tự nguồn tham chiếu cho cấp thấp hơn, có tối đa 16 cấp Tùy vào phiên bản, NTP hỗ trợ đến 256 Stratum Trong phiên NTP phát triển, dự kiến có stratum cho phép Hầu hết NTP clients tham chiếu đến Stratum server, nên không bị ảnh hưởng có cấp NTP timestamp Nhãn thời gian (timestamp) 64 bit NTP bao gồm 32 bit giây 32 bit phần chi tiết giây, NTP timestamp mô tả thời gian khoảng 232 giây (136 năm) độ chi tiết đến 2−32 (233 pico giây) NTP timestamp lặp lại 232 giây (136 năm) NTP lấy mốc thời gian vào tháng 1, năm 1990, lặp lại vào năm 2036, trước cố UNIX năm 2038 4/5 Network Time Protocol(NTP) Vì NTP hoạt động dựa chênh lệch time stamp không dựa giá trị tuyệt đối, ...Engineering Applications of Artificial Intelligence 17 (2004) 159–167 Evolving the neural network model for forecasting air pollution time series Harri Niska a, *, Teri Hiltunen a , Ari Karppinen b , Juhani Ruuskanen a , Mikko Kolehmainen a a Department of Environmental Sciences, University of Kuopio, P.O. Box 1627, Kuopio FIN-70211, Finland b Finnish Meteorological Institute, Sahaajankatu 20 E, Helsinki FIN-00880, Finland Abstract The modelling of real-world processes such as air quality is generally a difficult task due to both their chaotic and non-linear phenomenon and high dimensional sample space. Despite neural networks (NN) have been used successfully in this domain, the selection of network architecture is still problematic and time consuming task when developing a model for practical situation. This paper presents a study where a parallel genetic algorithm (GA) is used for selecting the inputs and designing the high-level architecture of a multi-layer perceptron model for forecasting hourly concentrations of nitrogen dioxide at a busy urban traffic station in Helsinki. In addition, the tuning of GA’s parameters for the problem is considered in experimental way. The results showed that the GA is a capable tool for tackling the practical problems of neural network design. However, it was observed that the evaluation of NN models is a computationally expensive process, which set limits for the search techniques. r 2004 Elsevier Ltd. All rights reserved. Keywords: Feed-forward networks; Time series forecasting; Parallel genetic algorithms; Urban air pollution 1. Introduction The forecasting of air quality is one of the topics of air quality research today due to urban air pollution and specifically pollution episodes i.e. high pollutant con- centrations causing adverse health effects and even premature deaths among sensitive groups such as asthmatics and elderly people (Tiittanen et al., 1999). A wide variety of operational warning systems based on empirical, causal, statistical and hybrid models have been developed in order to start preventive action before and during episodes (Schlink et al., 2003). In recent years, the considerable progress has been in the developing of neural network (NN) models for air quality forecasting (Gardner and Dorling, 1999; Koleh- mainen et al., 2001; Kukkonen et al., 2003). Despite the latest progress, there still exist some general problems that must be solved when developing a NN model. In the air quality forecasting, especially, the selection of optimal input subset (Jain and Zongker, 1997; John et al., 1994) becomes a tedious task due to high number of measurements from heterogeneous sources and their non-linear interactions. Moreover, due to a complex interconnection between the input patterns of NN and the architecture of NN (related to the complexity of the input and output mapping, the amount of noise and the amount of training data), the selection of NN architecture must be done simulta- neously. These aspects requires the formulation of search problem and the investigation of search techni- ques which are capable of facilitating model develop- ment work and resulting more reliable and robust NN models. In this context, the evolutionary and genetic algo- rithms (GA) (Holland, 1975) have proven to be power- ful techniques (Yao, 1999) due to their ability to solve linear and non-linear problems by exploring all regions of the state space and exploiting promising areas through genetic operations. The main drawbacks related to the using of GAs for optimising NNs have been Enriching Network Security Analysis with Time Travel Gregor Maier TU Berlin / DT Labs Robin Sommer ICSI / LBNL Holger Dreger Siemens AG Corporate Technology Anja Feldmann TU Berlin / DT Labs Vern Paxson ICSI / UC Berkeley Fabian Schneider TU Berlin / DT Labs ABSTRACT In many situations it can be enormously helpful to archiv e the raw contents of a network traffic stream to disk, to enable later inspection of activity that becomes interesting only in retrospect. We present a Time Machine (TM) for network traffic that provides such a capability. The TM leverages the heavy-tailed nature of network flows to capture nearly all of the likely-interesting traffic while storing only a small fraction of the total volume. An initial proof-of-principle prototype established the forensic value of such an approach, contributing to the investigation of numerous attacks at a site with thousands of users. Based on these experiences, a rearchitected implementation of the system provides flexible, high- performance traffic stream capture, indexing and retrieval, includ- ing an interface between the TM and a real-time network intrusion detection system (NIDS). The NIDS controls the TM by dynami- cally adjusting recording parameters, instructing it to permanently store suspicious activ ity for o ffline forensics, and fetching traf fic from the past for retrospective analysis. We present a detailed per- formance evaluation of both stand-alone and joint setups, and re- port o n experiences with running the system live in high-volume environments. Categories and Subject Descriptors: C.2.3 [Computer-Communication Networks]: Network Operations – Network monitoring General Terms: Measurement, Performance, Security Keyw ords: Forensics, Packet Capture, Intrusion Detection 1. INTRODUCTION When in vestigating security incidents or trouble-shooting per- formance problems, network packet traces—especially those with full payload content—can prove inv aluable. Yet in many opera- tional environments, wholesale recording and retention of entire data streams is infeasible. Even keeping small subsets for extended time periods has grown increasingly difficult due to ever-increasing traffic volumes. However, almost always only a very small subset Permission to make digital or hard copies of all or p art of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice a nd the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior s pecific permission and/or a fee. SIGCOMM’08, August 17–22, 2008, Seattle, Washington, USA. Copyright 2008 AC M 978-1-60558-175-0/08/08 $5.00. of the traffic turns out to be relevant for later analysis. The key difficulty is how to decide aprioriwhat data will be crucial when subsequently investigating an incident retrospectively. For example, consider the Lawrence Berkeley National Labo- ratory (LBNL), a security-conscious research lab (≈ 10,000 hosts, 10 Gbps Internet connectivity). The operational cybersecurity staff at LBNL has traditionally used bulk-recording with tcpdump to an- alyze security incidents retrospectively. However, due to the high volume of network traffic, th e operators cannot record the full traf- fic volume, which averages 1.5 TB/day. Rather, the operators con- figure the tracing to omit 10 key services, including HTTP and FTP data transfers, as well as myriad high-volume hosts. Indeed, as of this writing the tcpdump filter contains 72 different constraints. Each of these omissions constitutes a blind spot when performing incident analysis, one very large one being the lack of records for any HTTP activity. In this work we develop a system that uses dynamic packet filtering and buffering to enable effective bulk-recording Internet Time Synchronization: the Network Time Protocol 1,2,3 David L. Mills Electrical Engineering Department University of Delaware Abstract 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. Keywords: network clock synchronization, standard time distribution, fault-tolerant architecture, maximum- likelihood principles, disciplined oscillator, internet pro- tocol. 1. Introduction Accurate, reliable time is necessary for financial and legal transactions, transportation and distribution sys- tems and many other applications involving widely dis- tributed resources. How do hosts in a large, dispersed networking community know what time it is? How ac- curate are their clocks? In a recent survey involving 94,260 hosts of the Internet system, 20,758 provided local time using three time-transfer protocols [24]. About half of the replies had errors greater than two minutes, while ten percent had errors greater than four hours. A few had errors over two weeks. Most local clocks are set by eyeball-and-wristwatch to within a minute or two and rarely checked after that. Many of these are maintained by some sort of battery-backed clock-calendar device using a room-temperature quartz oscillator that may drift as much as a second per day and can go for weeks between manual corrections. For many applications, es- pecially distributed internet applications, much greater accuracy and reliability is required. This paper presents an overview of the architecture, protocol and algorithms of the Network Time Protocol (NTP) used in the Internet system to synchronize clocks and coordinate time distribution. The Internet consists of over 100,000 hosts on over 1500 packet-switching net- works interconnected by a similar number of gateways. In this paper the capitalized Internet refers to this par- ticular system, while the uncapitalized internet refers to any generic system of multiple networks interconnected by gateways. While the Internet backbone networks and gateways are carefully engineered for good service, op- erating speeds and service reliability vary considerably throughout the system. This places severe demands on NTP, which must deliver accurate and reliable time in spite of component failures, service disruptions and pos- sibly mis-engineered implementations. In the remainder of this introductory Section 1, issues in the requirements, approaches and comparisons with pre- vious work are discussed. The architecture of the NTP synchronization system, including the primary reference sources REPORT DOCUMENTATO ADA255 652 , ' m - Ill IIII 1! 11 III I11 IIi 11111 iIll " '- -' SUrtsemOue Ma"Wrese wE ted Ps unaumn I .AGENCY USE ONLY (Leew bIenbo 2. REPORT DATE 2.REPORT TYPE AND DATES COVERED 02 September 1992 FINAL REPORT 8/14/91-8/31/92 4. TILE AND SUBTITLE L FUNDING NUMBERS Neural Network Retinal Model Real Time Implementation (Neural Network Retinal Model) Contract IDAAHO1-91-C-R240 AUTHOR(S) Dr. Robert W. Means 7. PERFORMING ORGANIZATION NAME(S) A rEQA8 7 PERFORMING ORGANIZATION HNC, Inc. "'' REPORT NUMBER 5501 Oberlin Drive 3405-F-92 San Diego, CA 9212134 F 9SPONSORINGMONITORING AGENCY NAME(S) AND ADDRES(ES) 10. SPONSRINGJMONITORING Defense Advanced Research Projects Agency (DOD) AGENCY REPORT NUMBER 1400 Wilson Avenue I ArlIington, VA 22209-2308 11. SUPPLEMENTARY NOTES 12& DISTRIBUTIONAVAILABIUTY STATEMENT 12b. DISTRIBUTION CODE Unrestricted j 13. ABSTRACT (Mrxinma D worb) L, _ The solution of complex image processing problems, both military and commercial, are expected to benefit significantly from research onto biological vision systems. However, current development of biological models of vision are hampered by lack of low-cost, high-performance, computing hardware that addresses I the specific needs of vision processing. The goal of this SBIR Phase I project has been to take a significant I N neural network vision application and to map it onto dedicated hardware for real time implementation. The C neural network was already demonstrated using software simulation on a general purpose computer. During Phase I. HNC took a neural network model of the retina and, using HNC's Vision Processor (ViP) prototype hardware, achieved a speedup factor of 200 over the retina algorithm executed on the Sun SPARCstation. A performance enhancement of this magnitude on a very general model demonstrates that the door is open to a new generation of vision research and applications. The model is described along with the digital hardware implementation of the algorithm using the new ViP chip seL 14. SUBJECT TERMS 15. NUMBER OF PAGES Neural Network, Vision, Retina, Tracking, Real-Time, Hardware 23 II. PRICE CODE 17. SECURITY CLASSIFICATION 11. SECURITY CLASSIFICATION 1,. SECURITY CLASSIFICATION 20. UMITATION OF OF REPORT OF TM PAGE OF ABSTRACT ABSTRACT Unclassified Unclassified Unclassified Unlimited F" no" II S MW fm = t. *4i 92I9 16 SL 3 i I92 9 16 033 " I I Neural Network Retinal Model Real Time Implementation Acvuqaion For Final Report -I , : : . 3 2 September 1992 I_,_ Di tr! btIsn/ or i Av ti.abi11ty c' " - I Ail and/or .Dist Special Sponsored By: Defense Advanced Research Projects Agency (DOD) _ Defense Small Business Innovation Research Program ARPA Order No. 5916 Issued by U.S. Army Missile Command Under L,1-C QUATy rN CTe D 3 Contract # DAAH01-91-C-R240 HNC, Inc. Dr. Robert W. Means 5501 Oberlin Dr. 619-546-8877 San Diego, CA 92121 Neural Network Retinal Model .. .Network Time Protocol(NTP) Một phiên đơn giản NTP không cần yêu cầu lưu trữ thông tin trao đổi cũ gọi Giao thức Đồng Thời gian mạng Đơn giản - Simple Network Time Protocol hay... timestamp lặp lại 232 giây (136 năm) NTP lấy mốc thời gian vào tháng 1, năm 1990, lặp lại vào năm 2036, trước cố UNIX năm 2038 4/5 Network Time Protocol(NTP) Vì NTP hoạt động dựa chênh lệch time. .. theo RFC1305 cho Windows Time Service Tuy nhiên, Windows Time Service đảm bảo độ xác 1-2 giây Microsoft không đảm bảo không hỗ trợ xác dịch vụ W3 2Time nút mạng Dịch vụ W3 2Time không hỗ trợ đầy đủ