These are Smooth Streaming specific boxes, which provide duration and
time offset of subsequent(s) fragment(s), thus allowing us to avoid
manifest polling in "live" mode.

See the following file for reference:
http://download.microsoft.com/download/B/0/B/B0B199DB-41E6-400F-90CD-C350D0C14A53/%5BMS-SSTR%5D.pdfSigned-off-by: Jean-Baptiste Kempf <jb@videolan.org>