Let us first consider the case of a video stream addressed to one target only.

The path between the source and the target is automatically determined at run-time, by tracing the route via one dummy packet sent from source to target (it can be also predetermined by the VoD source according to some previously collected routing information). QoSNs move to the chosen hosts on the path and interrogate the AC database: if available resources are not enough for the desired QoS, QoSNs can coordinate and reduce their requests by scaling the stream (at the moment, by dropping frames in Motion JPEG streams or by reducing resolution in MPEG-2 ones). Only if these diminished reservation requests cannot be satisfied, the VoD service is denied. After a successful negotiation phase, the (possibly scaled) multimedia stream starts to flow.
During the video distribution, a link can fail or its quality can deteriorate, thus making impossible to a particular QoSN to maintain the negotiated quality. In that case, the interested QoSN can enhance the throughput of its link via stream striping on non-corouted paths. In this case, it sends back a message to temporarily stop the stream, and sends forward a message to suspend updates in AC tables on the path. Then, it sends its clones to handle new non-corouted paths and starts the negotiation phase with the clones. When negotiation completes, the QoSN sends back a message that restarts the stream: apart from a delay in receiving the stream, the VoD target goes on transparently.

In the case of multicast distribution of the same video stream (for N targets), the generated network traffic can be limited by exploiting location awareness of agents. While in traditional VoD systems the source generates N packet streams, one for each target, with the consequent overload, our approach can take advantage of locality. Our QoSNs can ascertain whether there are several targets within the same domain locality, and split packets only when it is necessary, in general only at the last hop. This is commanded by the QoSN at the gateway of the last domain.

See the next page for first performance evaluations of the MESIS VoD service.