CMCD
Common Media Client Data — the standard that lets the player tell m-cdn how playback is actually going.
What it is
CMCD (Common Media Client Data) is the CTA-5004 standard. The player attaches a small set of playback metrics to each media request — buffer length, measured throughput, requested bitrate, object type, and a session id — so the delivery side can see real client experience.
m-cdn parses the key fields (sid, cid, br, bs, bl, ot) off each request per CTA-5004 and feeds them into the Decision Engine.
Why it matters
Probes and CDN logs tell you about the network; only the player knows whether the viewer is actually rebuffering. CMCD is the signal that turns 'the CDN looks fine' into 'this viewer is struggling' — it is the highest-trust input the engine has. Without it the engine is half-blind.
How m-cdn ingests it
CMCD can arrive as query arguments (works everywhere), HTTP headers (cleaner, needs CORS), or batched Reporting Mode (lowest overhead, fewer players support it). The session id (sid) stitches one viewer's requests into a session for QoE analysis without ever identifying the person. For players that don't emit CMCD natively, mcdn-cmcd-enabler.js is a one-line embed that turns it on.
Standards & references
- CTA-5004 — Common Media Client Data