.ISM

Introduction

An .ism file is the server manifest of Microsoft Smooth Streaming. It sits on the streaming server next to the media files and tells the server which files hold which tracks at which bit rates. Players never read it directly: they ask for the address of the .ism file followed by /Manifest, and the server answers with the client manifest.

The media sits in fragmented MP4 files with the extensions .ismv for video (often with audio) and .isma for audio only. Servers such as Unified Streaming also use .ism files to serve the same media as HLS and MPEG-DASH.

Structure

The server manifest is a SMIL 2.0 XML document. The head holds meta elements, such as the name of the client manifest. The body holds a switch element with one video or audio element per encoding. Each one names its media file in src, gives its bit rate in systemBitrate, and identifies the track inside the file with a param named trackID.

<smil xmlns="http://www.w3.org/2001/SMIL20/Language">
  <head>
    <meta name="clientManifestRelativePath" content="film.ismc"/>
  </head>
  <body>
    <switch>
      <video src="film_3000.ismv" systemBitrate="3000000">
        <param name="trackID" value="1" valuetype="data"/>
      </video>
      <audio src="film_3000.ismv" systemBitrate="128000">
        <param name="trackID" value="2" valuetype="data"/>
      </audio>
    </switch>
  </body>
</smil>

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