NTP时间同步技术在中国联通信息化中的应用

在通信行业的信息系统建设中,对于时钟同步服务的需求越来越迫切。本文介绍了时钟同步技术（Network Time Protocol,简称NTP）的基本原理、工作模式和体系架构,并对NTP运行过程中的关键参数进行了分析和比较。通过搭建NTP系统,为中国联通的信息化建设提供了统一的时钟服务,在各系统建设中取得了良好的效果。     

基于NTP的时钟同步技术在成品油管道SCADA系统中的应用

本文介绍了成品油管道SCADA系统时钟同步的重要意义、NTP时钟同步工作原理,重点阐述了华南成品油管网SCADA系统时间同步应用现状、存在的问题,并提出了基于NTP时钟同步技术的解决方案。     

NTP协议在Y.1731中的应用研究

随着信息技术和网络技术的发展,人们对时间同步的精度要求越来越高。网络时间协议(NTP,Network Time Protocol)是一种基于UDP的时间同步协议。介绍了NTP协议的基本原理,Y.1731单端DM原理。通过NTP协议在Y.1731报文中的应用研究和实践,使传统的网络时间同步精度能达到毫秒级别。实验数据表明,文中所设计的网络时钟同步系统比传统的时钟同步具有更高的时钟同步精度。     

PTN时钟同步技术及应用

时钟同步是分组传送网（PTN）需要考虑的重要问题之一。可以采用同步以太网、IEEE1588v2、网络时间协议（NTP）等多种技术实现时钟同步。同步以太网标准的同步状态信息（SSM）算法存在时钟成环,以及难以对节点跟踪统计的问题。中兴通讯提出了一种扩展SSM算法可以改进时钟同步问题。在时间同步方面,由于NTP的精度还无法满足电信网的需求,仅采用1588v2又会带来收敛时间较慢、在网络负载较重时时间延迟精度容易受到影响等问题。中兴通讯提出了同步以太网基础的1588v2时间传递方案,对提高PTN网络中时间同步的精度起到了较好的作用。     

330kV变电站同步系统组网方式研究

文章通过对NTP/SNTP、IRIG-B和IEEE1588(PTP)三种时间同步协议进行精度、性价比和安全性三个方面的分析,提出了330kV变电站内同步系统组网方案和变电站与上级时钟的组网方案。     

测量互联网环境的单向延迟及不对称性研究

NTP协议(Network Time Protocol)的出现就是为了解决网络内设备系统时钟的同步问题。不幸的是,在通常的互联网环境中,数据传输的延迟不是恒定的,即使相同的路由,从NTP服务器到NTP客户端延迟与从NTP客户端到NTP服务器延迟,即单向延迟(OWD)不总是相同的。这对时间同步的准确性有很大的影响。目前广泛应用的PTP也同样存在这个问题。因此,为了提高时间的准确性,需要通过测量,提供有关实际传输OWD的时间分布和OWD的不对称性的研究。     

基于IPv6的SD-WAN部署

介绍了SD-WAN的发展背景及目前基于IPv4的SD-WAN方案，提出了将IPv6应用到SD-WAN场景的方案，分析了要实现基于IPv6的SD-WAN方案需要在现有underlay和overlay层面上做出改变，并对利用IPv6技术实现SD-WAN的可行性进行讨论。     

SD-WAN隧道技术与组网模式

介绍了SD-WAN技术现状,比较了SD-WAN相对传统专线的优势,描述了SD-WAN的基本技术特征,对SD-WAN的软件定义网络控制器、安全加密与租户隔离、SD-WAN业界标准进行了说明。通过分析与SD-WAN主流厂商相关的技术,对SD-WAN所用的隧道技术进行比较,对不同厂商选用的私有IPSec隧道、VxLAN over IPSec、NvGRE over IPSec、SSL隧道等组网方案做了分析比较,给出了优选的隧道技术方案。并进一步针对如何应用隧道技术实现组网,结合运营商组建SD-WAN的优势,提出优化意见,给出了POP点组网和点对点组网方案。最后对SD-WAN未来标准的制定、运营商与SD-WAN厂商的未来发展趋势进行了展望。     

以太网的时钟同步技术

随着分散控制的发展和系统范围的扩大,通过网络联系的分散控制节点之间的时间同步技术变得越来越重要。同步技术相当于一个网络系统的心脏,它为该系统中的每个模块传送正确的时钟信号。分析了目前以太网应用中的3类时钟同步协议：IEEE1588、NTP（网络时间协议）及SNTP（简单网络时间协议）,具体说明了IEEE1588、NTP及SNTP的基本原理,并对3类协议进行了简要的对比分析。     

SD-WAN关键技术

软件定义广域网(SD-WAN)是将软件定义网络(SDN)技术应用于广域网(WAN)连接的服务。重点阐述了SD-WAN应用的关键技术,包括SD-WAN服务中4种典型技术架构、SD-WAN的整体功能模块以及边缘设备中所采用的关键技术。与传统的WAN架构相比,SD-WAN技术以一种多接入、安全、策略驱动业务、弹性路由、多虚拟隧道、敏捷上云的方法重新定义了开放式的WAN架构。     

Network classless time protocol based on clock offset optimization

Time synchronization is critical in distributed environments. A variety of network protocols, middleware and business applications rely on proper time synchronization across the computational infrastructure and depend on the clock accuracy. The Network Time Protocol (NTP) is the current widely accepted standard for synchronizing clocks over the Internet. NTP uses a hierarchical scheme in order to synchronize the clocks in the network. In this paper we present a novel non-hierarchical peer-to-peer approach for time synchronization termed CTP-Classless Time Protocol. This approach exploits convex optimization theory in order to evaluate the impact of each clock offset on the overall objective function. We define the clock offset problem as an optimization problem and derive its optimal solution. Based on the solution we develop a distributed protocol that can be implemented over a communication network, prove its convergence to the optimal clock offsets and show its properties. For compatibility, CTP may use the packet format and number of measurements used by NTP. We also present methodology and numerical results for evaluating and comparing the accuracy of time synchronization schemes. We show that the CTP outperforms hierarchical schemes such as NTP in the sense of clock accuracy with respect to a universal clock.     

基于线性规划的Internet端到端时延的估计

测量Internet端到端时延特征是研究Internet端到端分组行为的重要内容之一,它能够应用于QoS(Quality of Service),SLA(Service Level Agreement)的管理、拥塞控制算法研究等许多方面.常用的端到端时延测量方法大多依赖于GPS接收机或采用NTP协议来实现收发端时钟的同步,但由于GPS接收机价格较高不可能每台主机都能配备, NTP协议的精度不能满足要求。该文基于线性规划的方法估计收发时钟的频差、相对时钟偏差等参数,以获得端到端时延的估计。作者在几条不同的链路上进行了测试,结果表明该方法能有效消除收发时钟不同步的影响。     

SD-WAN在中国移动国际网络中的应用

软件定义广域网(SD-WAN)是将SDN技术应用到广域网场景中所形成的一种服务,用于降低互联成本,提升业务部署灵活性.本文简要分析了SD-WAN技术优势,重点介绍了SD-WAN在中国移动国际网络中的应用解决方案,并从多角度给出了国际网络拓展SD-WAN建设的后续发展建议.     

Improved algorithms for synchronizing computer network clocks

The Network Time Protocol (NTP) is widely deployed in the Internet to synchronize computer clocks to each other and to international standards via telephone modem, radio and satellite. The protocols and algorithms have evolved over more than a decade to produce the present NTP Version 3 specification and implementations. Most of the estimated deployment of 100000 NTP servers and clients enjoy synchronization to within a few tens of milliseconds in the Internet of today. This paper describes specific improvements developed for NTP Version 3 which have resulted in increased accuracy, stability and reliability in both local-area and wide-area networks. These include engineered refinements of several algorithms used to measure time differences between a local clock and a number of peer clocks in the network, as well as to select the best subset from among an ensemble of peer clocks and combine their differences to produce a local dock accuracy better than any in the ensemble. This paper also describes engineered refinements of the algorithms used to adjust the time and frequency of the local clock, which functions as a disciplined oscillator. The refinements provide automatic adjustment of algorithm parameters in response to prevailing network conditions, in order to minimize network traffic between clients and busy servers while maintaining the best accuracy. Finally, this paper describes certain enhancements to the Unix operating system kernel software in order to realize submillisecond accuracies with fast workstations and networks     

On synchronization techniques: performance and impact on time metrics monitoring

The paper presents an experimental comparison of device synchronization strategies to monitor quality of service (QoS) time metrics, such as the one‐way delay and the delay variation. The compared strategies are based on global positioning system (GPS), public Internet network time protocol (NTP) servers and ad hoc GPS‐based NTP servers with different treatments of the NTP traffic. The presented experimental results allow to quantitatively evaluate the level of performance achievable using each synchronization technique. Finally, considering a satellite test bed, we focus our analysis on two relevant aspects of time metrics monitoring: the impact of the synchronization errors on the metrics observation and the different values that can be measured when a metric is monitored at the application or data link level. Copyright © 2003 John Wiley & Sons, Ltd.     

基于D-S的时钟同步竞争安全算法

针对物联网时钟同步安全研究的不足,提出了一种时钟同步安全算法.首先,基于投票竞争的理念,给出了时钟同步安全算法过程;然后,为了解决此过程中涉及到的网络通信延迟不确定难题,提出了基于D-S理论的解决方法;最后,进行了仿真测试实验,结果表明在网络通信延迟不确定情况下,所提同步算法能容忍内部节点攻击,提高了时钟同步的安全性.     

PTN Clock Synchronization Technology and Its Applications

Clock synchronization is an important issue for packet transport networking. Current clock synchronization technologies include synchronous Ethernet, IEEE 1588v2, and Network Time Protocol (NTP). However, individually, these technologies are beset with certain problems. Synchronization Status Message (SSM) algorithm for synchronous Ethernet standards suffers clock ring, and has difficulty tracing and counting nodes. An extended SSM algorithm can improve clock synchronization. NTP is too imprecise to meet the requirements of telecom networks, yet IEEE 1588v2 alone can lead to slow convergence time and influence time delay precision when the network is heavily loaded. ZTE therefore proposes an IEEE 1588v2 solution based on synchronous Ethernet in order to effectively raise the precision of Packet Transport Network (PTN) time synchronization.