中文摘要：

随着网络的迅速发展，网络流量不断增大，各种攻击日趋频繁，在这样的环境下，网络所提供的服务并不是时刻都能保证处于可用状态，需要一个服务评价指标来衡量服务可用性。利用态势评估方法，从节点、系统两个层次进行分析，定义了针对给定特定的系统服务的态势分析函数，从而构建了一个针对网络服务可用性的监测评估方法。该方法能够提供针对某一个特定服务的直观态势分析，并且可以看到系统资源使用情况，使其对系统的安全威胁状况有具体的了解。通过在正常和受DDOS攻击网络中进行实验，验证了该方法的有效性。

英文摘要：

The increasing use of the Internet and attacks make it necessary to develop techniques that analyze network and service availability. Monitoring and quantifying network component behavior is key to make network reliable and robust. By using situation analysis, this paper proposes an approach to network service availability evaluation which is aimed at certain systems. The contribution of this paper is two-fold： First,key-factors and secondly, the system. This approach can provide a direct situation analysis to the certain services and can also analyze the system resource usage,thus havs a concrete and whole scope of the system situation. The effectiveness of this approach is approved by the large amounts of experiments in both normal and DDOS attacked network.