Described embodiments include methods and techniques for creating media container files. An exemplary method comprises steps wherein, instead of estimating the size of a media segment header based on the segment duration, identified media data boxes are used. In the method, an identified media data element is written in a container file. Included into the identified data element, an identifier is used as a reference for the identified media data of the segment by other elements. The identifier can be a first movie fragment sequence number, or a first track fragment decode time.