Modelling Cache Expiration Dates Policies in Content Networks. Content Caching Problem Formulation

Introduction Content networks are usually virtual networks based over the IP infrastructure of Internet or of a corporative network, which use mechanisms to allow accessing a content when there is no fixed, single, link between the content and the host or the hosts where this content is located. Even more, the content is usually subject to reallocations , replications, and even deletions from the different nodes of the network [6][7][9]. In the last years many different kinds of content networks have been developed and deployed in widely varying contexts: they include peer-to-peer networks, collaborative networks, cooperative Web caching, content distribution networks, subscribe-publish networks, content-based sensor networks, backup networks, distributed computing, instant messaging, and multiplayer games. As a general rule, every content network is actually a knowledge network, where the knowledge is the information about the location of the nodes where each specific content is to be found: this is "meta-information", in the sense of being the information about the information contents themselves. The objective of the network is to be able to answer each content query with the most complete possible set of nodes where this content is to be found; this corresponds to discover the content location in the most effective and efficient possible way. As both nodes and contents are continuously going in and out of the network, the task of maintaining updated the network meta-information is very difficult and represents an important communication cost. In this context, cache nodes are used to hold the available meta-information; as this information is continuously getting outdated, the cache nodes must decide when to discard it, which means increasing communication overhead for the sake of improving the quality of the answers. The policy employed for determining these cache expiration dates have a large impact in the performance of a network; this problem is related but at the same time different to other problems involving caches, as in this case the cached information does not take too much place, but gets outdated very quickly (usually in caching problems the cached information uses up significant space, and is valid for more extended time periods). In this work, we model different policies for fixing the cache expiration dates for the information about the contents' location; in particular, we give mathematical programming formulations which represent the case where the cache expiration dates are equal for all contents, the case where the cache expiration dates are …