Bringing the Web to the Network Edge: Large Caches and Satellite Distribution

In this paper we discuss the performance of a Web caching distribution where caches are interconnected through a satellite channel. Web caching is emerging as an important way to reduce client-perceived latency and network resource requirements in the Internet. A satellite distribution is being extensively deployed to offer broadcast services avoiding highly congested terrestrial links. When a cache-satellite distribution is used, documents requested for the first time in any cache are immediately broadcast to all other caches via the satellite. Caches connected to the satellite distribution end up containing all documents requested by a huge community of clients. Thus, the probability that a client is the first one to request a document is very small, increasing the number of requests that are hit in the cache. In this paper we develop analytical models to calculate upper bounds in the performance of a cache-satellite distribution. We derive simple expressions for the hit rate in the caches as well as the bandwidth in the satellite channel and the required capacity of the caches. Based on our model we find that an ISP with a large cache connected to a cache-satellite distribution can theoretically achieve hit rates up to 85%. We calculate that the satellite channel needs a bandwidth of several Mbps to transmit newly appearing documents as well as document updates. Additionally, we use trace driven simulations to validate our model and evaluate the performance of a real cache-satellite distribution. Our results show that an ISP with a cache of 64 Gbytes connected to a satellite channel can achieve local hit rates of up to 65%. Preliminary results of this study were presented at the ACM/IEEE MobiCom’98 Workshop on Satellite-based Information Services (WOSBIS’98), Dallas