On-line content delivery and vertical alliances between conduit and content providers are nowadays crucial issues in on-line content markets. In this paper, we discuss and compare two types of model for on-line content delivery: push and pull. We assume non-zero marginal cost for network transits, justified by the presence of network services for content delivery (like data caching). Under both models, we show that the rationale behind vertical strategic integration between conduit and content providers in the case of successive monopolies.