Based on the previous research on the portal crane administration , one standpoint put forward in this paper is idea that is on the foundation of reliability in this paper , two viewpoints take shape when reason is analyzed , that is fatigue break and appearing the flaw that will extend . what is the fatigue break ? ( this phenomenon is always happen in some place ) , the fatigue is that board appears apophysis and concave when the board is pressed . so that the board ability of bearing the weight of load will descend 本论文建立在已有的港口装卸机械管理的研究基础之上，进一步提出了以可靠性为基础的港口装卸机械的管理思想，已有的研究表明，港口机械设备破坏主要由于两个原因，第一是稳定性问题，特别是构件的局部稳定性，所谓稳定性是指：在板的平面内，板受到平行于板面的压力作用，使板发生隆起、内陷等凹凸不平的结果，从而使板受载能力减弱；第二是结构出现疲劳裂纹及其扩展，这是由于设备长时期运行，而造成设备某些部位出现裂纹并得以扩展，使设备承载能力下降。
As a part of the national 863 planned project - " 2001aa411320 regional modern integrated manufacture system development and application oriented city chengdu , deyang and mianyang " and the science and technology key project of sichuan province - " the engineering configuration intelligent cad technology oriented the process of design " , this thesis proposes a modified scheme , which restrains efficiently the " feedback implosion " question , and is able to realize the reliable multicast . this scheme has considerably academic and practical significance 作为国家863计划项目： “ 2001aa411320以成德绵为核心的区域现代集成制造系统开发及应用”与四川省重大科技攻关项目： “ 01gg010 - 01区域制造业信息化关键技术与应用研究”的一部分，本文提出的方法可以有效抑制“反馈内陷”问题，能够实现ip多播的可靠传输，为协同设计系统提供可靠的多点通信支持，具有一定的理论和实用价值。
This thesis discusses detailedly the flow process of ip multicast , error discovery method and error recovery method , and also proposes a new error control method to implement reliable multicast in computer supported cooperative design system : serial integrates with timer are used to discover error , arq mechanism based on nak feedback is modified to correct error , " sliding window " mechanism is imported to make up the drawbacks of traditional nak , and " slotting and damping " technique is used to resolve the question of " feedback implosion " which results from multicast reliability control 详细研究了ip多播传输的流程、数据传输的差错检测方法及差错恢复方法，提出在协同设计系统中实现可靠多播的控制方法：利用序列号和计时器相结合的方法进行传输数据的差错检测，改进了基于nak反馈信息的arq重传机制来实现差错恢复，并引入了滑动窗口机制来解决传统nak方法的不足，采用“时间分槽?抑制”技术来解决由于进行多播可靠控制而产生的“反馈内陷”问题。
