A computer-implemented method is provided for transparently optimizing data transmission between a first endpoint and a second endpoint in a computer network. The endpoints have a directly established data session therebetween. The data session is identified by each endpoint at least to itself in the same way throughout the session. The method includes the steps of: relaying data between the endpoints transparently in the session using a network optimization service; and transparently modifying or storing at least some of the data transmitted from the second endpoint to the first endpoint using the network optimization service in order to optimize data communications between the endpoints, wherein transparently modifying at least some of the data comprises changing the data, replacing the data, or inserting additional data such that the first endpoint receives different data than was sent by the second endpoint.