Terminology

This specification uses a few terms that are easy to mix up in real-time video systems. This page defines them precisely.

Core Terms

Term	Meaning in this spec	Why it matters
Capture frame	A single screen image sampled by the Sentinel (pre-encode).	Controls what the user could see and when.
Encoded sample	The encoded representation of a capture frame in the H.264 stream (what MP4 carries as a sample).	This is what is actually transported and stored.
Keyframe (IDR)	An H.264 IDR frame (Instantaneous Decoder Refresh). A decoder can start cleanly from here.	Controls join/switch latency for a new Proctor.
Initialization segment	fMP4 `ftyp` + `moov` bytes for the session. Contains codec config (SPS/PPS), resolution, timescale. Contains no frames.	Must be appended to MSE before any media data can be decoded.
Fragment	A small fMP4 media unit (`moof` + `mdat`) produced frequently and pushed live. Contains one or more encoded samples.	Controls live latency and how often Proctors see updates.
Join fragment	A fragment whose first sample is an IDR keyframe (random access point).	A Proctor should start playback from a join fragment.

Do not equate keyframe interval with delivery latency.

Keyframe interval controls how quickly a new viewer can join without a server-initiated on-demand keyframe.
Fragment cadence (how often fragments are produced and pushed) controls how quickly an existing viewer sees new video.

Practical Implications

Real-Time Requirement

Live streaming MUST push fragments continuously. A “20 second” value can apply to the maximum keyframe interval (join point spacing), but MUST NOT imply that media is only delivered every 20 seconds.

What “Sent As It Is Created” Means

When this spec says “sent as it is created”, it means:

A fragment is finalized (it already contains at least one encoded sample).
That finalized fragment is immediately sent over WebSocket.

It does not mean “start sending an empty 20s segment at t=0”.

Last updated on March 16, 2026 • J.H.F.

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