BT The objective of Pre-Congestion Notification (PCN) is to protect the quality of service (QoS) of inelastic flows within a Diffserv domain in a simple, scalable, and robust fashion. This document defines the two metering and marking behaviours of PCN-nodes. Threshold-metering and -marking marks all PCN-packets if the rate of PCN-traffic is greater than a configured rate ("PCN-threshold-rate"). Excess- traffic-metering and -marking marks a proportion of PCN-packets, such that the amount marked equals the rate of PCN-traffic in excess of a configured rate ("PCN-excess-rate"). The level of marking allows PCN-boundary-nodes to make decisions about whether to admit or terminate PCN-flows. [STANDARDS-TRACK]