The paper examines the challenges of defining and measuring availability to support real-word service comparison and dispute resolution through SLAs. We propose a rigorous and unambiguous definition of availability in cloud services. In the light of this, we show that what appear to be apples-for-apples comparisons between real-world SLAs are often based on ambiguous definitions, and even where SLAs are well defined, they differ significantly in their interpretation of availability. We show how two example real-world SLAs, would lead one service provider to report 0% availability while another would report 100% for the same system state history. On the basis of this, the paper concludes by arguing for the importance of standardising availability definitions and examines which elements need to be standardised and, just as importantly, which do not. Many of the results of this paper can be generalised to service level attributes other than availability: in general, such standard service definitions are a key element of a true commodity market in cloud resources, allowing service comparability before purchase, redress in the case of failure to deliver expected value and enhancing accountability in the supply chain.

We propose a new stream authentication scheme that is suitable for live packet streams distributed over a lossy channel, such as an IP-multicast group. Packets are signed together in a block and the recipient can authenticate this block if the loss rate per block is lower than a certain threshold, which can be chosen based on the characteristic of the communication channel. This scheme provides both integrity and non repudiation of origin, and in a majority of situations, it performs with less overhead in bytes per packet than previously proposed practical live stream authentication schemes.

In this article we propose a new framework for multicast security based on distributed computation of security transforms by intermediate nodes. The involvement of intermediate nodes in the security process causes a new type of dependency between group membership and the topology of the multicast network. Thanks to this dependency, the containment of security exposures in large multicast groups is ensured. The framework also ensures both the scalability for large dynamic groups and the security of individual members. Two different key distribution protocols complying with the framework are introduced: an extension of the EIGamal encryption scheme, and one based on a multiexponent version of RSA.

In this paper we propose a new framework for multicast security based on distributed computation of security transforms by intermediate nodes. The involvement of intermediate nodes in the security process causes a new type of dependency between group membership and the topology of the multicast network. Thanks to this dependency, the containment of security exposures in large multicast groups is assured. The framework also assures both the scalability for large dynamic groups and the security of individual members. Two different key distribution protocols complying with the framework are introduced. The first protocol is an extension of the El Gamal encryption scheme whereas the second is based on a multi-exponent version of RSA.