The LOC Header Extensions carry optional metadata for the corresponding LOC Payload. The LOC Header Extensions are contained within the MOQ Object Header Extensions. This metadata provides necessary information for end subscribers, relays and other intermediaries to perform their operations without accessing the media payload. For example, media switches can use this metadata to perform their media switching decisions without accessing the payload which may be encrypted end-to-end (from original publisher to end subscribers).
The following sections define specific metadata as LOC Header Extensions and register them in the IANA registry for MOQ Object Header Extensions.
Other specifications can define other metadata as LOC Header Extensions and register them in the same registry. Each extension must specify the following information in the IANA registry.
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Name: Short name for the metadata (not sent on the wire)
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Description: Detailed description (not sent on the wire)
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ID: Identifier assigned by the registry (varint)
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Length: Length of metadata Value in bytes (varint if ID is odd, omitted if ID is even)
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Value: Value of metadata (varint if ID is even, Length bytes if ID is odd)
2.3.1. Common Header Data
2.3.1.1. Capture Timestamp
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Name: Capture Timestamp
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Description: Wall-clock time in microseconds since the Unix epoch when the encoded media frame was captured, encoded as a varint.
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ID: 2 (IANA, please assign from the MOQ Header Extensions Registry)
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Length: Varies (1-8 bytes)
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Value: Varies
2.3.2. Video Header Data
2.3.2.1. Video Config
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Name: Video Config
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Description: Video codec configuration "extradata", as defined by the corresponding codec specification, which maps to the WebCodecs VideoDecoderConfig description property in the EncodedVideoChunkMetadata.
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ID: 13 (IANA, please assign from the MOQ Header Extensions Registry)
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Length: Varies
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Value: Varies
2.3.2.2. Video Frame Marking
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Name: Video Frame Marking
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Description: Flags for video frames which are independent, discardable, or base layer sync points, as well as temporal and spatial layer identification, as defined in [Framemarking], encoded in the least significant bits of a varint.
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ID: 4 (IANA, please assign from the MOQ Header Extensions Registry)
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Length: Varies (1-4 bytes)
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Value: Varies
2.3.3. Audio Header Data
2.3.3.1. Audio Level
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Name: Audio Level
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Description: The magnitude of the audio level of the corresponding audio frame as well as a voice activity indicator as defined in section 3 of [RFC6464], encoded in the least significant 8 bits of a varint.
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ID: 6 (IANA, please assign from the MOQ Header Extensions Registry)
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Length: Varies (1-2 bytes)
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Value: Varies