hang

hang is the media format on top of moq-lite: a catalog track that describes the media tracks, and a container that gives each frame a timestamp. It is modeled on WebCodecs so a browser can decode it directly. The spec is draft-lcurley-moq-hang. Broadcast names end in .hang so a player knows which catalog to expect.

Catalog

catalog.json is a JSON track listing the renditions of each media kind and the decoder config for each. It updates live as tracks come and go, and a compressed twin (catalog.json.z) is published alongside it.

{
  "video": {
    "renditions": {
      "hd": {
        "codec": "avc1.64001f",
        "description": "0164001f...",
        "codedWidth": 1280,
        "codedHeight": 720,
        "container": { "kind": "legacy" }
      }
    }
  },
  "audio": {
    "renditions": {
      "en": { "codec": "opus", "sampleRate": 48000, "numberOfChannels": 2, "container": { "kind": "legacy" } }
    }
  }
}

Each rendition extends the WebCodecs VideoDecoderConfig or AudioDecoderConfig. Every WebCodecs codec is fair game; H.264, H.265, VP8, VP9, AV1, AAC, and Opus are what the tools produce today. Properties shared by every video rendition (display size, rotation, flip) sit at the section root.

A few things the catalog can express beyond decoder config:

• Renditions in another broadcast. A rendition may point at a relative broadcast path, so a transcoder can publish a ladder that adds low rungs and references the source's original rendition without re-publishing its bytes.
• Stalled renditions. A publisher can flag a rendition as temporarily bad so players prefer another one without the track disappearing.
• Timelines. A rendition may name a small timeline track mapping every group to its start time, which is what lets the HLS gateway build playlists without subscribing to media.
• Extensions. The root is a loose object. Applications add their own sections (scte35, chat, transcript) next to video and audio, optionally naming a track that carries the data. Every library exposes a way to write your section without clobbering the media ones, and readers ignore what they don't know.

Container

The container.kind on each rendition says how frames are framed:

Kind	Frame layout	Use
legacy	varint microsecond timestamp + codec payload	The default. Cheapest.
cmaf	moof + mdat	fMP4 passthrough for HLS/DASH interop; ~100 bytes per frame.
loc	small property block + payload	The IETF LOC container.

A consumer skips renditions with a kind it doesn't recognize and carries them through when republishing the catalog.

Groups and keyframes

A video group is a GoP: it begins with a keyframe and holds the frames that depend on it. That alignment is what makes MoQ's congestion behavior safe. A relay can drop a whole group, a viewer can join at any group boundary, and the decoder never sees a frame whose reference is missing. Audio groups are independent too and typically hold about a second.

The description field carries out-of-band codec setup (an avcC box for H.264). When it is absent, the parameter sets ride inline before each keyframe, which is what avc3/hev1 tracks do. Decoders should handle both.

Your own format

hang is a convention, not a requirement. If you control both ends, publish whatever frames you like on raw tracks; the relay never looks inside them. The MoQ Boy demo mixes hang media tracks with raw JSON status and command tracks on the same broadcast.