CMCD, or Common Media Client Data (CTA-5004, 2020), is a Consumer Technology Association standard that lets a streaming player attach a compact, standardized set of key-value fields to each media request it already makes to the CDN, as an HTTP header or query argument. The reserved keys are exactly the quality signals a QoE dashboard needs, encoded bitrate, buffer length, a buffer-starvation flag set when the player just rebuffered, measured throughput, a startup flag, top playable bitrate, and a session GUID that ties thousands of CDN log lines into one session. Because it rides the CDN's own logs, it lets you reconstruct the bitrate and stall sequence at the delivery layer with no separate analytics SDK, and by design it carries no IP, cookie, or location. Its catch in v1 is that cadence is tied to segment requests; the 2026 revision CTA-5004-A (CMCDv2) adds an event mode that decouples telemetry from segments. Its server-side mirror is CMSD.
