.ISMC

Introduction

Smooth Streaming is Microsoft’s adaptive streaming protocol, introduced in 2008 with IIS Media Services and the Silverlight player. Like HLS and MPEG-DASH it cuts video into short fragments at several qualities. The client manifest, an .ismc file, is what the player downloads first: it lists the tracks, the qualities of each and the timing of every fragment.

The server usually sends the client manifest at an address ending /Manifest, built from the server manifest and the media files. The media is stored as fragmented MP4 in .ismv (video) and .isma (audio) files.

Structure

The manifest is XML with the root element SmoothStreamingMedia, whose attributes give the version and the total duration. Times are counted in units of 100 nanoseconds unless a different time scale is given.

Each StreamIndex element describes one track, such as the video or the audio in one language. Its Url attribute is a template for fragment addresses, like QualityLevels({bitrate})/Fragments(video={start time}). Inside it, one QualityLevel element per encoding gives the bit rate, codec and, for video, the picture size, and a run of c elements gives the duration of each fragment.

<SmoothStreamingMedia MajorVersion="2" MinorVersion="0" Duration="1200000000">
  <StreamIndex Type="video" Chunks="60" QualityLevels="2"
      Url="QualityLevels({bitrate})/Fragments(video={start time})">
    <QualityLevel Index="0" Bitrate="3000000" FourCC="H264" MaxWidth="1280" MaxHeight="720" CodecPrivateData="…"/>
    <QualityLevel Index="1" Bitrate="1000000" FourCC="H264" MaxWidth="640" MaxHeight="360" CodecPrivateData="…"/>
    <c d="20000000"/>
    <c d="20000000"/>
    …
  </StreamIndex>
</SmoothStreamingMedia>

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