Each segment is coded as a base-tier (BT) chunk, and multiple enhancement-tier (ET) chunks.
BT chunks:
represent the entire 360 view at a low bit rate and are pre-fetched in a long display buffer to smooth the network jitters effectively and guarantee that any desired FOV can be rendered with minimum stalls.
ET chunks:
Facebook 360 video:
https://code.facebook.com/posts/1638767863078802
Assessment:
https://code.facebook.com/posts/2058037817807164