2.1 An Example Server

 

We implemented a video-clip server that delivers video clips interactively to clients. Examples of such a system include museum kiosks, retail services (e.g., mall, information resources), educational services (e.g., library and encyclopedia), and entertainment services. For instance, upon entering a museum, information about exhibits, resources, and staff can be retrieved from computers located at kiosks throughout the museum.

A prototype version of the video-clip server system is built using Windows 95 PC clients running the ActiveMovie application to view videos and a miss-server that runs on Linux 2.0 to retrieve the videos. Our instructional server provides small video clips (varying from about 20 seconds and 4 MB to about 180 seconds and 50 MB each) to its clients.

The Windows 95 clients use Lava's reliable object protocol to communicate with the hit-server. The protocol is implemented as a Windows kernel extension (a so-called Vxd element) that communicates directly with the network driver using Window's NDIS network-driver interface. A special ActiveMovie source filter was built that transfers ActiveMovie requests into Lava's get transactions. The ActiveMovie source filter requests a series of video blocks that correspond to a consecutive intervals of the video. The size of each block is 32 KBytes.

The according miss-server executes as a user process on Linux. Lava's reliable object protocol (see Section 2.4) is incorporated into the Linux kernel to enable hit-server/miss-server communication. Application-specific policy is added to: (1) download video objects into the miss-server from the Web using (unmodified) HTTP and (2) implement a custom cache-replacement policy that controls the hit-server.

In conjunction with the mentioned video-clip miss-server, the hit-server can serve up to 402 clients simultaneously with different video clips (see Section 4).