A significant challenge that arises if the buffer manager allows seek operations is the following: When can we be certain that a data item will not be accessed anymore in the future? With a purely sequential stream format — e.g., an MPEG-2 program stream — the read pointer in the data buffer will generally be monotonically advanced and data behind it can be freed. However, with the AVI file format seek operations do not guarantee a monotonically increasing read position. Obviously, if read locations are randomly distributed across the complete file then having a limited playout buffer will be impossible.

Luckily, seek and read operations exhibit a localized behaviour. Figure 3 shows the trace data we obtained from a 10 minute long DivX AVI movie. After initially reading the frame table at the very end of the file, all operations take place within a \'sliding window\' of a few megabytes of data.

To accommodate this behaviour we implemented a sliding release pointer in the following way. A maxReadOffset pointer keeps track of the furthest seek/read position. This pointer only moves forward, i.e., a seek backwards will leave it unchanged.

A freeOffset pointer is then made to follow a fixed distance behind the maxReadOffset, for example by 4 megabytes.