Benefits to the network should be the end result of performance evaluation. This means that average bandwidth savings, for example, should be replaced with bandwidth savings at peak link, or network, utilization. For example, a cache that thrashes during such times would provide zero bandwidth expansion, no matter what the average traffic reduction it delivers may be. Availability is the probability that a request submitted to the network will lead to the successful initiation of service. Availability depends on the length of the downtime caused by a cache failure, until any fail-safety mechanism kicks in to restore service.
Reliability is the probability that a service, once successfully initiated, runs to completion.
When cached files are small, and hence transaction lifetimes are short, reliability can be safely ignored. This is especially true if availability is high. The converse is not true. Many of these failures occurred after the download started. Aside from the tactical effects of network caching, which can be quantified reasonably well using the approach we outlined above, we should not ignore the strategic impact on the network.