Multimedia Distribution (1)

The SOMA Video on Demand (VoD) service is realized in terms of a set of mobile lower-level services, implemented by agents distributed over the paths between the source and the targets of the video stream. The SOMA VoD permits users to require a QoS level for any multimedia stream, and allows to manage and adjust the requested quality during service provision, to respond to dynamic modifications of network resource availability.

The service is realized by coordinating two different types of SOMA management agents: the QoS Negotiators (QoSNs) that define and grant a specific level of quality for the service, and the Admission Controllers (ACs) that manage the resources to be engaged by local intermediate nodes (see the Figure).


Figure. Tunneling, corouting and multicast in the VoD service
(click here for a larger view of the picture)

ACs have been realized by tailoring already implemented agents for general-purpose monitoring. ACs are present on every node of the network; this assumption is not severe because they are implemented by mobile agents that can move and be installed whenever they are needed. Each AC manages locally available resources and keeps track of resources currently committed to already accepted streams. The flow specifications of streams are recorded in a local table of tuples. Any tuple represents the statistics of VoD traffic between the local and the receiving host: the first time, it contains values calculated upon a short sample of communication; then, it is updated by monitoring real traffic of current VoD sessions.

ACs are in charge of answering to reservation requests from QoSNs. The VoD service requires the coordination of a set of QoSN agents located at the source, at the target and at some intermediate nodes. These agents can also move from node to node by need. QoSNs maintain session state: they record user preferences and flow specifications for a video stream. QoSNs evaluate the feasibility of meeting these requirements against the local AC database and exploit the SOMA communication facility to perform the negotiation phase for the definition of the achievable QoS. After the negotiation phase, during multimedia streaming, any QoSN is in charge of receiving packets from the previous QoSN and of forwarding them to the next QoSN. When multiple video streams interest the same network node, one QoSN can handle all of them.

 
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