CableLabs Ericsson ZTE Corp. The Simple Two-Way Active Measurement Protocol (STAMP) enables one- way and round-trip measurement of network metrics between IP hosts, and has a facility for defining optional extensions. This document updates the definition of the Class of Service TLV (originally defined in RFC 8972) to enable the measurement of manipulation of the value of the Explicit Congestion Notification (ECN) field of the IP header by middleboxes between two STAMP hosts, and to enable discovery and measurement of paths that provide differential treatment of packets depending on the value of their ECN field.