  The demand and usage of 360$^{\circ}$ video services are expected to
increase. However, despite these services being highly bandwidth intensive, not
much is known about the potential value that basic bandwidth saving techniques
such as server or edge-network on-demand caching (e.g., in a CDN) could have
when used for delivery of such services. This problem is both important and
complicated as client-side solutions have been developed that split the full
360$^{\circ}$ view into multiple tiles, and adapt the quality of the downloaded
tiles based on the user's expected viewing direction and bandwidth conditions.
This paper presents new trace-based analysis methods that incorporate users'
viewports (the area of the full 360$^{\circ}$ view the user actually sees), a
first characterization of the cross-user similarities of the users' viewports,
and a trace-based analysis of the potential bandwidth savings that
caching-based techniques may offer under different conditions. Our analysis
takes into account differences in the time granularity over which viewport
overlaps can be beneficial for resource saving techniques, compares and
contrasts differences between video categories, and accounts for uncertainties
in the network conditions and the prediction of the future viewing direction
when prefetching. The results provide substantial insight into the conditions
under which overlap can be considerable and caching effective, and inform the
design of new caching system policies tailored for 360$^{\circ}$ video.
